gdk-pixbuf-file-info (filename)

filename -- a string with the name of the file to identify

format -- a gdk-pixbuf-format instance describing the image format of the file or nil if the image format was not recognized
width -- an integer with the width of the image, or nil
height -- an integer with the height of the image, or nil

Parses an image file far enough to determine its format and size.