**NB.** This answer applies to the PC/Mac/… version of the game; while this question is tagged for XBox the title/question is likely to get hits beyond that specific version of the game, so for those people here is their answer:

As [SevenSidedDie][1] writes: you cannot add a cape yourself to your account.

But with mods you can in game. The latest (for MC 1.5.1) [CoFHCore][2] adds in game commands to add capes to players.

    /cofh cape set <URL>

And the linked forum topic includes a cape image template [here][3].


  [1]: https://gaming.stackexchange.com/a/114851/39111
  [2]: http://www.minecraftforum.net/topic/1773528-151-team-cofh-mods-thermal-expansion-now-with-hd-skins-and-capes/
  [3]: http://www.minecraftforum.net/topic/1773528-151-team-cofh-mods-thermal-expansion-now-with-hd-skins-and-capes/page__st__40#entry22057008