Really, it is very easy!
All that is necessary is, that the image, you want to place is already online somewhere on any server.
If this is not the case and the image is only on your computer up to now, don't worry, there is a simple solution to that as well further down.
just copy the full address of the image in the message-box and add an exclamation mark in front and behind it.
It should look more or less like this:
just go here: http://tinypic.com/ (it's a place, where you can upload images for free), and upload your image.
On the confirmation screen, copy the text from the third box (Link:), past it in my message box and put exclamation marks around it.
It should look more or less like this:
As the presents have a certain width, your image should be maximum 425 pixels wide.