Check in the tech support forum for more possible options if you'd like but your signature has to be one picture itself I believe. So you can use a website like picnik.com or image editing program like photoshop to combine the pictures you want into one image file. I did mine at picnik.com.
I used photobucket.com. They have a very easy signature creator tool and you can choose multiple photos in one sig. It combines everything into one picture and then gives you the link to put in your sig.