Step 1: Find an image host. Photobucket works well, try Freephotohost. If you have comcast or any other broadband provider, they should give you a file locker and it gets easy to get pictures to the internet.
Step 2: Make sure you know the url of your picture, and use this html in your message:
img src="(url of the image here)" ( inside "" brackets. And that's all there is to it.