With an uncompressed image format, like BMP, any partial digital data you can correctly recover from a file will directly translate into pixels you can recover.
With an RLE compressed file, it is almost the same, though there are some exceptions about uniformly colored (or maybe patterned) areas in the image – unlikely to be found in a photograph though.
With JPEG and other “complex” compression types, the digital data in the file is strongly interdependent – some pieces of data will make ABSOLUTELY no sense to you or any software unless another piece of data is also available. This will worsen recoverability.
Both formats are well documented publicly. If there is significant value potentially lost, try to get some knowledgeable and/or professional (data recovery firm, systems administrator, computer programmer) to help you.
If a BMP file seems to be completely unreadable, verify that it isn’t still a JPEG file that some horse renamed to .BMP!