You can assign any picture as the background to an email.
You should use this feature with caution, of course, since it will increase the size of the email and if you choose a really obnoxious image it will also annoy whoever you are sending the email to.
Open up a new email, click on the Options tab and then the Page Color drop-down button, and then the Fill Effects item at the bottom of the menu.
You can use the Textures or Patterns on the various tabs if you like Or you can choose a particular picture from the Picture tab by using the Select Picture button to browse and find a file.
You'll want to make sure to choose an image that can be tiled, and also make sure that it's small since it's going to be included in the email.