The announcement Reddit thread contained a Reddit employee mentioning that they already have a cache of the majority of Imgur images because they've been using it to create thumbnails Reddit displays alongside the headlines.
Apparently they've been storing the full size images too and not throwing it away after creating thumbnails (wise move).
It could - SomethingAwful basically did that when ImageShack got annoying. They mirrored all the linked images to their own servers, and the community came up with a new host - WaffleImages
