Are you sure the image is a PNG? Just because a URL ends in .png, the file does not have to be a PNG. Browsers and image viewers generally accept anything that roughly looks like an image, more or less ignoring file extensions, URL parts, and MIME types.

Check the first few bytes: They should be 0x89 0x50 0x4E 0x47 0x0D 0x0A 0x1A 0x0A, according to both Portable_Network_Graphics and http://www.libpng.org/pub/png/spec/1.2/PNG-Contents.html.

Alexander