How to check if a page is displaying a specific <img> tag.

Question

What is the best way to determine if a page on a website is REALLY displaying a specific img tag like this <img src=http://domain.com/img.jpg&gt;? A simple string comparison is easy to fool using http comments <!-- -->. Even if the html tag exists it could be deleted with JavaScript. It could also be obscured by placing an image over it using CSS. Do you know of a solid method of detecting the img tag dispute these obscuring attacks listed? Do you know of another method of obscuring the image? Python code to detect the image would be ideal, but if you know of a good tactic or method that will earn you a +1 from me.

Answer 1

The only surefire way I can think of is to render the page and check. It is simple to strip comments etc. But if scripts are involved, it is not possible to have a general solution that will not amount to executing them (I believe this is the first time I ever invoked Church's theorem...).

Answer 2

You could place a script anywhere that processes the request, counts the view and delivers the image like this:

http://yourhost.com/imageprocess?image=media/foo/bar.jpg

Then you can be sure that the image was loaded. If if was viewed, you of course can't be sure, however.

Answer 3

I don't think you can ever be sure. First, you're not even sure the program will stop.
Aside from that, consider the following scenarios. Your <img> can be added, removed or get obscured using JavaScript, CSS and/or server-side:

• randomly.
• at specific times.
• to a certain part of the world.
• according to differences and bugs between browsers.

Google is facing a similar problem - people are hiding search keywords in hidden text and links to get a better rank. Their solution is to penalize sites with hidden text. They get away with it because they're Google; people depend on them for traffic.
As for you, you can't do much better than to ask nicely...