bitmap-spec-pbitmap-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.