Others have already pointed you at some image modules, but the way I read the question is that you have people inputing an URL for an image which you're then including inline.

 If that is the correct interpretation then you aren't going to have any joy with the modules; if the image isn't upon your server then you can't easily get it's size - you'd have to download a copy from the remote site and work on that.

 If this is for a forum or script of some kind you might be better allowing the user to upload an image to your site and then using that local copy. That way you could filter the image on size/dimensions.

 If you want to go this way make sure you use strict, use taint, and look at the CGI module for file uploads.

Steve
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steve.org.uk