I currently have a gaping hole in my schedule and can deliver this for you very quickly.
I have code using MutationObserver that I wrote to intercept <script> tags before before they are executed, this could easily be adapted to your project. This is the best option performance wise. Otherwise it is possible to intercept the HTTP reply.
A couple of notes:
1) I'm not sure what you mean by location on disk. If you're looking for location in cache, this may be a problem. I'm not sure about other OSs but on chrome images are no longer stored as discrete files on disk, which makes sense for performance reasons.
2) I'm not sure I can make any guarantees about intercepting the images before rendering using MutationObserver. I can guarantee that they will be intercepted before any other javascript runs, which *should* amount to the same thing, but I would have to look into the threading model. It would be helpful to understand exactly what you want to do, and why you have this requirement, so I can determine what your options are.
I'm looking forward to hearing from you,
Calin B.