Its not hard at all. First, you already know how to put text in your sig, so the hardest part is done.
Next, just select the image you want, crop to an appropriate size (usually somewhere around 50-200px tall and 300-800 wide). Then you need a website to host the image on the internet for you. I prefer photobucket.com. You can register there for free and host plenty of pictures.
After you have uploaded the image to them, they will give you web addresses for the image. You need to copy the address for forums, it will begin and end with "IMG" in square brackets. Then just paste that text in your signature here. After you save it, you should see the image along with your text
I hope that is clear enough, just post back if you need more info.