Recover broken images that are corrupt

possibly with some software expertise. BMPs use no compression, or very simple RLE compression, so it might be possible to repair the images one by one with some loss of information. Were they converted by you to "bmp" format?

Have you tried copying one of the files, changing the file extension to .jpg (or whatever type of file they were before you had the problem) and seeing if they will then open properly? Some data recovery programs will misidentify what type of file a photo is. For instance, I once had to recover data that had been accidently deleted from a drive. The recovery software identified all of the Canon raw files as tiffs and attached the tiff file extension. All I had to do was change the file extensions back to .cr2 and they functioned perfectly again.

Underlying theory:

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.

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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.

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If a BMP file seems to be completely unreadable, verify that it isn't still a JPEG file that some horse renamed to .BMP!