I'm on T-Mobile in the UK. This is a T-Mobile mobile broadband dongle.
They do give you the option to get the original image. They do this sub-optimally by injecting javascript into every page I visit. They hijack the alt-text, replacing it with a message that says something like "Shift R improves the quality of this image, Shift A improves the quality of all images on this page".
The IP address they use for the caching / mangling proxy is 1.2.3.X (where X tends to be 9 or 11) - I'm not making that up, they actually use 1.2.3.9 which is a bad idea.
(http://i.imgur.com/DYbVSbj.png)