Unable to change custom skins

This is a confirmed bug and it is currently worked on.

Edit: Anything downloaded from the internet and saved to the camera roll retains its format, so .png stay as a .png, while anything taken from the camera is saved as a .jpeg.

The bug however, is caused when the game tries to access the file system. As iOS 6 does not support copying files from the camera roll to an app's sandboxed directory, Minecraft Pocket Edition thus comes up with the "This is not a skin, silly." which is a generic error.

Note: The error in-game does not specify the cause, but the inferred cause would be "file not found".

For now, all you can do is wait...

As for Facebook, for example able to use an image (or multiple images) from the camera roll - That's due to the image not being saved to disk, but instead, transmitted; loaded to memory and then uploaded through the same method Safari does so.

Update: See this question for information on how to change your skin.

Update: Now I remember: It was a comment on the bug tracker.

To change your Minecraft Skin manually on iOS, simply open a file system viewer and browse to the following folder:

.../[Minecraft PE]/Documents/game/com.mojang/minecraftpe/

And name it: custom.png.

Different devices is the problem I think.

It's possible that you're using a different version, or it could be a too old skin that's almost but not quite the same as the second one, or it could be you need to backup your iPad, or the app you get it from.

I recommend checking all the possibilities in the following order:

1. Make sure your Minecraft version is up to date.
2. Check if there is an update for your skin, or try a different skin that has been certified for the latest Minecraft version;
3. In your device settings, look for "iCloud" and make a backup, then restore from that;
4. Try a different app.

I hope one of these works!