Someone posted a link to a great archive of these things, but they were all jpegs without option to download the original bitmap images. Don't convert old bitmaps to jpegs!

Unfortunately a lot of the 'sources' that may get posted in this thread will likely have porn (i.e. BBS dumps, early 90s shovelware discs that pried from BBSes for the same). but they're the primary survivng hoard of the Amiga/ST pictures you're after (rather than a dork's favorite barrage of late 90s animated gifs which may get misposted in this thread)

(also by porn I definitely don't mean certain traced up Bakshi artwork - I mean digitized, obviously Bush-era, color scanned stuff from certain magazines.)

The Internet Archive made https://gifcities.org/, a search engine specifically for old internet gifs,
If you're looking for a cool collection, Cameron's world https://www.cameronsworld.net/ is a cool interactive 90s style website with music, pictures and its own mock browser "catscape"!

cd.textfiles.com