How "compressed" are the sprites?
Some sort of Run-length_encoding algorithm
was used to compress "empty" (transparent?) parts of images.
If I deleted everything but the image data, would I have an image or would I have to do something else?
If by "everything but the image data" you mean only headers, the answer will be "no". But if you'll delete headers, delete control sequences of commands (inserted to compressed sprite by original compressor) and insert missed transparent blocks then you will get the uncompressed image data.
What do you mean by "simple cases like CREDITS"? Is this just about the number of images contained in each files, or is there something more going on?
No, not the number of images in one file. Simple cases are cases where original image did not contain transparent pixels. Like CREDITS and some other rectangular pictures.
For such images compressor still added some control sequences of command, but did not remove anything. So this files could be viewed in Texture Finder or GBS (that I personally use) even in "compressed form". In those tools control sequences of commands will look like vertical lines (look at my screenshot in previous post).
P.S. I didn't completely understand the algorithm of compression so far, so it could be that some sprites are compressed with some other tricks in addition to RLE.
P.P.S. first sprite from file BL1SHEEP.IMB. Compressed size = 11999 bytes. Decompressed size = 20736. After decompression: