The problem I fear is that bitmaps do use padding (4 bytes per line), so that probably won't work.

Just 'cause I'm curious, are you saying every bitmap format aligns each line of pixels to 4 bytes?
Or that, regardless of the length of a line, there are 4 empty bytes at the end of each one?

And is the first bit or the last bit of a 16-bit 555 bitmap pixel the ignored bit?

Not every bitmap format, only the Windows BMP format AFAIK.

Every line of pixels is padded to a full multiple of four bytes. The padding bytes are added at the end of a line. 16-bit bitmaps are always stored as 5-5-5, xRRRRRGGGGGBBBBB, unless the color masks in the header say otherwise. For simplicity sake the color masks in the sample hex dump are set to 0, then xRRRRRGGGGGBBBBB is the default.

The problem I fear is that bitmaps do use padding (4 bytes per line), so that probably won't work.

Just 'cause I'm curious, are you saying every bitmap format aligns each line of pixels to 4 bytes?
Or that, regardless of the length of a line, there are 4 empty bytes at the end of each one?

And is the first bit or the last bit of a 16-bit 555 bitmap pixel the ignored bit?

Every line of pixels is padded to a full multiple of four bytes. The padding bytes are added at the end of a line. 16-bit bitmaps are always stored as 5-5-5, xRRRRRGGGGGBBBBB, unless the color masks in the header say otherwise. For simplicity sake the color masks in the sample hex dump are set to 0, then xRRRRRGGGGGBBBBB is the default.