Sorry, I made a mistake in the code I posted earlier. The properties passed to Valentin's ImagXtra should be strings, not symbols, so the relevant line should have read:
PUT ix.ix_saveImage(["image": tMem.image, "filename": mPath & tName & ".png", "format": 13])
I have made a little utility projector executable that automates this, and allows selection of multiple cast libraries, all in one fell swoop. It seems to export your bitmaps successfully. If you want a copy, PM me your address and I'll email it to you (~2.5MB)
