I would definitely check is_error first, but as mentioned before many times a missing file won't properly return a 404 code. Normally this is because the webmaster put a full URL in the ErrorDocument directive, which causes Apache to send a 302 response. What I've done before iss checked the file size. I could be fairly confideent that the images would be 80K or more, while an error page isn't likey to be more than 10K-20K. So anything under 50K is assumed an error, anything over 50K is assumed to be the image.