serve cached data based on the freshness of an URL argument

[Rares Vernica]

Hello,

I am new to Varnish so so the question might me a bit poorly stated. I am
trying to configure Varnish so that serving an URL from cache depends on
the freshness of another URL given as argument to the original URL.

For example, the original URL might look like:

  http://fancyprocessing.com/processimage?http://images.com/123.jpg

If I have seen this URL before and cached its result, I would like
Varnish to use the cached data only if:

   http://images.com/123.jpg 

has not changed since the result was cached.

Any pointers would be greatly appreciated.

Thanks!
Rares