A PNG file does have image dimensions.
[link|http://www.libpng.org/pub/png/spec/PNG-Chunks.html#C.IHDR|PNG image header.]
4.1.1. IHDR Image header
The IHDR chunk must appear FIRST. It contains:
Width: 4 bytes
Height: 4 bytes
Bit depth: 1 byte
Color type: 1 byte
Compression method: 1 byte
Filter method: 1 byte
Interlace method: 1 byte
Obviously one cannot display an unknown image file format, but an image file that does not include image dimensions isn't practical unless it is, by definition, fixed.
Alex
"No man's life, liberty, or property are safe while the legislature is in session."\t-- Mark Twain