If you've extracted the files, try to load them in an image viewer that identifies files by their content rather than filename. ACDSee for Windows is one option. If that fails, Google for "graphics file formats identifiers" or similar to get lists of ways to identify the files, then open in a hex editor.
There's a good chance they're IFF ILBM, but there's also a good chance they're in a custom format, in which case it'll be impossible without a lot of experimenting and/or reverse engineering. If they are in a custom format, it's likely to be (interleaved) bitplanes with widths divisible by 16 pixels. Don't expect any kind of chunky format.
Often multiple images are stored in collection files, in which case a ripping tool like this one
may help, although only if they're in a recognisable format. If not, the way the collection file is organised could give you clues too. You could look for what seem to be lists of file offsets, dimensions/depth, and so on.
And finally you may find the data is compressed. That makes it harder, especially if it's done with a custom packer, which would force you to look at the executable code for the unpacking routines. An easy way to test if the files are packed is to pack them (with say, WinZip). If you get very little compression, they were almost certainly compressed already. If you get good compression, chances are good the images are uncompressed (or at least compressed with a basic algorithm like RLE).
But regardless, there's always WinUAE, screen captures and paint packages. :)