First, I can assume that all urls that end with jpeg, jpg, bmp, png or gif are images, and others aren't.
I thought of, and tried two solutions:
Matching the regular expression .(jpe?g|bmp|png|gif)$
Using ends-with to check each separately
But, it appears that neither of these exist in XPath 1.0, or at least, they don't exist in Firefox (I am writing a greasemonkey script, so it is only important for the path to work in Firefox).