They are from Corel Professional Photo CDs on the Internet Archive. It's a huge set of PhotoCD disc images.
Corel made four collections of 200 discs in total. The Internet Archive has mostly complete of set 1 (produced 1993-94) and set 2 (produced 1994-95). Unfortunately, between the missing discs in the sets they have, and the two other collections, there are probably 450 discs still out there which aren't online. It's a shame since there are some titles that would be perfect for this thread.
To get the photos: I download the ISO, unpack it, and use a program to convert the uncompressed .PCD files to jpeg.