bitmap-spec-p is a built-in function in `C source code'.

(bitmap-spec-p OBJECT)

Value is non-nil if OBJECT is a valid bitmap specification.
A bitmap specification is either a string, a file name, or a list
(WIDTH HEIGHT DATA) where WIDTH is the pixel width of the bitmap,
HEIGHT is its height, and DATA is a string containing the bits of
the pixmap. Bits are stored row by row, each row occupies
(WIDTH + 7)/8 bytes.