Establishing the size of an image is straighforward in php, you use the getimagesize command.

The problem is that if you checked the image size for every [img] tag for each new post then it would slow the forums down. In fact, this solution wouldn't be 100% foolproof because someone could replace the image (on the web) with a larger one once the validation had been passed.

Therefore you would really have to check every image size every time a page that contains images is displayed. This would be horrendous!

Pete