Known derivative images:
bear, polarbear: Reference photo by Amareta Kelly (https://www.flickr.com/photos/42409752@N07/6266114045/), licensed CC-BY 2.0
gloves_cloth: Reference SVG from Algot Runeman (runeman.org), via openclipart.org; distributed under CC-0.
ape: Original is from RLtiles, a CC-0 collection of pixel art. Modified here under CC-0.
wolf, dwolf: Derived from base wolf & dwolf, which are derived from RLtiles, a CC-0 collection of pixel art. Modified here under CC-0.
bone_drake: Original is from RLtiles, a CC-0 collection of pixel art. Modified here under CC-0.
elf_lass: Derived from clsc elf_lass, which is in turn derived from RLtiles, a CC-0 collection of pixel art. Modified here under CC-0.
messageboard, messageboard_red: Referenced base image messageboard/messageboard_red, licensed under CC-0.
dragonclaw_dagger: Derived from base image dragonclaw_dagger, licensed under CC-0.
file: Derived from base image file, licensed under CC-0.
kite_shield: Derived from base image kite_Shield, under CC-0.
coif: Created by SilverNexus, referencing base image coif. Licensed as CC-0.
lantern, lantern2, lantern_off, lantern2_off: Derived from art by David Gervais. Licensed CC-BY-3.0.
yellow_[0-9], yellow_[A-F], yellow_ne, yellow_nw, yellow_se, yellow_sw: Derived from art by David Gervais. Licensed CC-BY-3.0.
bones3: Redrawn from a David Gervais tile. Licensed CC-BY-3.0.
charcoal: Derived from Saiapatsu's base set charcoal image, licensed CC-0.
misc/go/point, stone_(white|black): Derived from art by Kevin Zheng, licensed CC-BY-3.0.
handmirror: Referenced base set handmirror, licensed CC-BY-4.0.

Derivatives of unknown licensing:
capitalhold_west: Referenced from base image capitalhold_west, of unknown licensing.
out_tower: Referenced from base image out_tower, of unknown licensing.
pagoda_tower_east: Referenced from base image pagoda_tower_east, of unknown licensing.
hovels: Referenced from base image hovels, of unknown licensing.
l_shop1: Referenced from base image l_shop1, of unknown licensing.
rl_house3, rl_house_west: Referenced from base image rl_house3, of unknown licensing.
university: Referenced from base image university, of unknown licensing.
treefort, treefort2, treefort3, treehouse: Referenced respective base set images, and derived from retro tree_3, all of unknown licensing.
archblack, archgold, archgreen, archtree, archwood: Referenced respective base set images, all of unknown licensing.
brazier, unlit_brazier, firepot: Referenced base image brazier, of unknown licensing.
lampost: Referenced base image lampost, of unknown licensing.
broken_bird_gen: Derived from retro bird_gen, of unknown licensing.
broken_scorpi_gen: Derived from scorpi_gen, of unknown licensing.
horse: Derived from base image horse, of unknown licensing.
broken_behold_gen: Derived from retro behold_gen, of unknown licensing.
broken_devil_gen: Derived from retro devil_gen, of unknown licensing.
broken_dragoncave, broken_dragoncave_blue, broken_dragoncave_green: Derived from retro dragoncave, of unknown licensing.
ice_warrior: Derived from retro angel_knight, of unknown licensing.
broken_giant_gen: Derived from retro giant_gen, of unknown licensing.
broken_gnoll_gen: Derived from retro gnoll_gen, of unknown licensing.
clown: Referenced from base clown, of unknown licensing.
evilclown: Referenced from base evilclown, of unknown licensing.
nun: Referenced from base nun, of unknown licensing.
broken_bee_gen: Derived from retro bee_gen, of unknown licensing.
chest_1_mimic: Derived from retro chest_1, of unknown licensing.
broken_pixie_gen: Derived from retro pixie_gen, of unknown licensing.
fire_troll: Derived from retro troll, of unknown licensing.
broken_sarcophagus: Derived from retro vampiregen, of unknown licensing.
broken_skull: Derived from skull_gen, of unknown licensing.
bag_empty: Derived from retro bag_popcorn, of unknown licensing.
vial_empty: Derived from retro vial_yellow (or any other), of unknown licensing.
wbottle_empty: Derived from retro water, of unknown licensing.
potion_empty: Derived from the retro potion styling, all of unknown licensing.
crossroad: Referenced from base crossroad, of unknown licensing.
monument: Referenced from base monument, of unknown licensing.
store_food, store_food_barn: Derived from *really* old monochrome food shop image, of unknown licensing.
woodwindow_0, woodwindow_1: Derived from retro woodwall and the style of retro wwindow, both of unknown licensing.
awindow_0, awindow_1: Derived from retro woodwall and the style of retro wwindow, both of unknown licensing.
redcwindow_0, redcwindow_1, westcwindow_0, westcwindow_1, dwindow_0, dwindow_1, dwindow_2, dwindow_3: Derived from retro cwall and the style of retro wwindow, both of unknown licensing.
flagstone_win1: Derived from retro flagstone and the style of retro wwindow, both of unknown licensing.
stwindow_0, stwindow_1, stwindowblue_0, stwindowblue_1, stwindowmossy_0, stwindowmossy_1, stwindowred_0, stwindowred_1: Derived from retro stwall and the style of retro wwindow, both of unknown licensing.
window_0, window_1: Derived from retro wall and the style of retro wwindow, both of unknown licensing.
b_axe1: Derived from base image b_axe1, of unknown licensing.
dhaxe_1: Derived from base image dhaxe_1, of unknown licensing.
dhaxe_2: Derived from base image dhaxe_2, of unknown licensing.
shaxe_1: Derived from base image shaxe_1, of unknown licensing.
handcrossbow, handcrossbow_bone: Derived from base image handcrossbow, of unknown licensing.
reflexbow, reflexbow_bone: Derived from base set reflexbow/reflexbow_bone, both of unknown licensing.
sflail_1: Derived from base image sflail_1, of unknown licensing.
kama_1, kama_2, scythe_1, t_star1, twinblade_spear: Derived from respective base images, all of unknown licensing.
giants_knife, hooksword, rapier, butterflysword_1, butterflysword_2, vlongsword, sicklesword: Derived from respective base images, all of unknown licensing.
altarixalovh, altarsori, altarvalk, mbaltar_none: Derived from David Sundqvist's altar graphics, of unknown licensing.
ipond: Derived from retro grasspond, of unknown origin.
rain1, rain2, rain3, rain4, rain5: Derived from base image rain puddles, of unknown licensing.
broken_gravestone: Derived from retro gravestone, of unknown licensing.
ranger: Derived from retro elf, of unknown licensing.
footpath_[0-F]: Derived from base image footpath_[0-F], of unknown licensing.
bloodreaver: Derived from base image bloodreaver, of unknown licensing.
mbworkbench, mbtanbench: Referenced from their non-mapbuilding counterparts; see tanbench, workbench
mbfirepot, mbbrazier: Referenced from non-mapbuilding counterparts; see firepot, brazier
tanbench, workbench: Derived from table_4, of unknown licensing.
rl_house2: Referenced from classic image rl_house2, of unknown licensing.

Original images with known licenses:
jade_coin: Created by SilverNexus. Licensed as CC-0.
amber_coin: Created by SilverNexus. Licensed as CC-0.
dresser, dresser2, dresser2_cw, dresser_cw, close_dresser, close_dresser2, close_dresser2_cw, close_dresser_cw: Created by SilverNexus. Licensed as CC-0.
punch_dagger: Created by SilverNexus. Licensed as CC-0.
rain, snow_c: Created by SilverNexus. Licensed as CC-0.
pl_dragon_*, dragon_stoned: Created by SilverNexus. Licensed as CC-0.
knife, fork, spoon, utensils: Created by SilverNexus, only alteration from base version is canvas size. Licensed as CC-0.

Images with attribution of author, but no known license:
altardevo, altargaea, altargnarg, altarvalr, altargoro, altarlyth, altarmostr, altarnone, altarrugg: David Sundqvist (added 0.93.3)

Known problem images:
g_worm, purple_worm: These are ripoffs of Final Fantasy worms, and should be replaced.
GreatDemon, medium_demon, fiend: These are ripoffs of the demon designs from 1994 DOS game Dark Legions. GreatDemon is basically a Dark Legions demon posterized to 3 colors, and medium_demon and fiend were derived from GreatDemon.
housesmall: Appears to be derived from the home icon here: https://www2.ccs.neu.edu/research/demeter/course/projects/demdraw/www/index.html. No known license on the image.
giantrat: Appears to be derived from an animated rat gif of questionable licensing.

Needs more research:
burnout: Found at https://web.archive.org/web/19981201235415/http://www.ameritech.net:80/users/schmitz/TROOP33.html under the name radioact3.gif, dating back to December 1998. Does not appear to be origin of it, merely a use. More investigation needed to find origin.