What you have described is definitely doable with the new WebExtensions API. The hardest problem would be to determine the URL of the full size image, if it's not in the HTML source somewhere. We are having a WebExtensions workshop in about a month. I will bring this type of extension there as a suggestion, what the participants can do.
Michal
P.S. If you need it jsut for IMDb, try https://addons.mozilla.org/firefox/addon/imdb-plus/, maybe it will help.
