yes indeed, i just encountered this problem.
so may i know what version of php r u using?
i suggest u use at least PHP 4.2, to make those function available
and work u need download some file from GD website and compile it under *nix.
below instruction is from php manual
You can use the image functions in PHP to get the size of JPEG, GIF, PNG, SWF, TIFF and JPEG2000 images, and if you have the GD library (available at )http://www.boutell.com/gd/