http://social.technet.microsoft.com/Forums/en-US/outlook/thread/f8ad2a46-5153-4237-a3c2-a94b9d9aabe9/

If you want to use AutoCorrect to put the picture in automatically as you type, then:

Create a new message and insert the picture as you want it.
Then select the image and paragraph in which you inserted it.
Now click File > Options > Editor Options button.
Then click Proofing > AutoCorrect Options.
In the Replace box type the text you want to type to insert the current selection (your picture).
Click OK until back in the message.

Now type your text, defined above, and press the Space bar. The picture should now appear replacing the text.

Worked for me.