Since it's only been an hour, we'd recommend waiting a bit longer. It shouldn't take longer than a few hours at most (and may be longer for higher res formats). If nothing changes, try to delete and reupload. Tweet back if it's still not working after the second upload attempt.
-
This Tweet is unavailable.
Loading seems to be taking a while.
Twitter may be over capacity or experiencing a momentary hiccup. Try again or visit Twitter Status for more information.