Save Content From An Iframe To A File?

We use the services of an external company in our website. We load this content in via an iframe. Some of this content happens to be an image.

This image is only stored on the external company's site for a limited time. We need this image to be stored on our servers somehow. Because the content is loaded dynamically from another site it's a little more difficult to access via a server side script
I'm thinking it would go something like:

load in the html for the iframe into a variable,
(no idea how yet, I don't think 'include' works like that)
then scan through the file for the link to the image,
(would be easy with javascript selectors)
then request that url from them and save it as a file somewhere.

I can't think how i can do this easily with php nor ajax... any ideas?
This should work:
maybe something like, use javascript to find the element url, then send a request to another page with a server side script to handle saving the url
...but I'm thinking there might be a better way of doing it.

The people who made the application I'm supposed to be maintaining/extending didn't care for users with no javascript (i know, i know), so because other key functionality doesn't work without javascript (at current...but I'm rebuilding the whole thing at some point but this is a priority feature for asap) i'm not worrying about making sure it works for users without javascript.