Re: What is the difference between "raw" image and "bit map" image?
Which image format can be read by a FPGA design - completely depends on your design. In so far the question is rather pointless, or are you asking about a specific design?
A meaningful question could be which image format is suitable to be processed without much design overhead.
If you manage to implement a decompressor in your FPGA, you can of course use a compressed format . If JPEG is too complex, you may want to refer to simple run length encoding, or basic 1D compression methods that are used in fax transmission.
Uncompressed bitmap files like bmp have still a header that describes the format details. If you strip the header, you have a raw bitmap. *.bmp is always a good starting point.