As far as I am concerned, no link outside of the stack exchange network is rot proof. Even links to content on beta sites within the stack exchange network could be subject to the site being closed, so it is always a good idea to make sure that your answers are fully formed and self contained.
Self-contained answers are one of the founding principles of Stack Overflow, the culture of encouraging self-contained answers is a large part of what makes stack exchange sites more useful than OSQA, Askbot or other QA sites which have tried to emulate stack exchange.
Note that I would consider stack.imgur.com to be part of the network. I have no idea what the commercial relationship is between Stack Exchange and Imgur, but even if the latter did close down, Stack Exchange would probably host that content elsewhere and rewrite all of the references.
See Your answer is in another castle: when is an answer not an answer? for more information on the network wide perspective.