Just roll over the toolbars at the top of the message window. There is an icon for "insert image" and basically follow the directions from there. You will need to make sure that the image is the correct size before inserting it, but the directions will help.
If it is the picture beside your name it is called an avitar and you can change or add this by using the usercp at the top of the page but as Judy says you have to make sure its not to big.And please keep it apropriate to the site!
catmando!