The imgur service itself has nothing to do with Stack Exchange. Anyone who wants to use it to host/share images for their own purposes is free to do so.
In fact, that's exactly what the SE team did for a long time. It wasn't until recently that they cut a deal with imgur to get a dedicated server and a custom sub-domain (
So, I don't really understand why you would need or want to use the custom sub-domain when you could just use the regular imgur service like everyone else. It's completely free, and it should be just as reliable and stable as the custom