Eric Raymond documents two already-existing functions (bmp, png).

Doc/lib/libimghdr.tex

@@ -28,6 +28,8 @@ return value from \function{what()}:
   \lineii{'rast'}{Sun Raster Files}
   \lineii{'xbm'}{X Bitmap Files}
   \lineii{'jpeg'}{JPEG data in JIFF format}
+  \lineii{'bmp'}{BMP files}
+  \lineii{'png'}{Portable Network Graphics}
 \end{tableii}
 
 You can extend the list of file types \module{imghdr} can recognize by