Use valid images for the test PNG and GIF data
    
    Replace the data in make_png() and make_gif() with small but valid
    images from https://github.com/mathiasbynens/small
    
    In the case of PNG in particular, the file command now returns "data"
    for a PNG header with no IHDR chunk. The new test data allows make_png
    to output files that are still recognizable as PNG.