Photoshop RAW exports 24-bit, unless you have non-opaque pixels, then it can export 32-bit. If you need 32-bit raw bitmaps you could export a 32-bit uncompressed TGA, and rip out the headers and footer.
Or leave in the headers and footer, since TGA is an incredibly simple format that was designed to basically encapsulate packed pixel data that could be blitted to a framebuffer.
Again this depends if we're trying to do VGA "hello world" or if we're trying to actually parse an image file format. In the former case, trying to parse headers just adds extra room for bugs and makes it harder to figure out where something's going wrong (i.e. "is the problem with copying the data into the framebuffer, or are we actually copying the wrong data in the first place?").
Also, I don't know about Photoshop, but GIMP is able to export as either "standard" (interlaced - with each successive byte giving the red, green, and blue values in turn) format or planar (where all the red values are given first, then all the green values, then all the blue values) format. The latter could be useful for some VGA modes which use a planar framebuffer. There doesn't seem to be an option to save the alpha channel even if the image contains transparent areas (although one could always save it separately by extracting the alpha channel as a greyscale image and combining the resulting raw image files, which would take a bit of work but should be fairly simple for anyone competent in C and/or with experience in processing raw image data).