mmm lets see, to post a pic you need the url code, so get that code you need to upload the photo on a page like this one http://imageshack.us/ so you upload the picture then the web page gives you the code and then you paste the code where you want
To get to your signature:Hover over preferences at the top of your page (next to inbox), and click change signature .
Then find a picture you want to use. Right click the image and click view image, then copy the code in the address box.
Go back to your signature box and put this code