KGLoad comes with some screenshots in iff format. IIRC there are other packs for it, but I cannot find at the moment.
non-iff images packs:
GameBase Amiga v1.6 Screenshots ( png )
Open Amiga Game Database ( FS-UAE Game Database )
Lemonade screenshot packs
with
XnView you can convert those non-iff images to iff format ( batch converting for a whole folder ).