Well, there are many ways to do it. One of which is to get yourself an account with a site that allows you to store pictures, such as photobucket.com. When you save pictures into your account there, you get an html tag that you can use if you want to add the picture somewhere, like on your website if you have one, or as part of your signature here. If you go to edit signature and paste the code in there, it should work. Keep in mind that there is a limit to how big pictures can be though. Keep it under that limit, and it should work really well 🙂