iMessage altered on iPad iPhone
It is very difficult to answer from your description. The most likely reasons are:

a) You forgot that you sent it.

b) You sent it without knowing that you did so.

c) Someone borrowed one of your devices and send it on your account.

d) You gave your iMessage password to someone, or they guessed it, and they sent the message on your account.

It is much more unlikely that it happened due to Apple employees manipulating your iMessages or NSA staff doing the same - and much more unlikely than bugs in the iMessage system letting other people's images appear as being sent from you.