Welcome to Talk Chelsea.
To get a picture in your signature you first need the picture to be hosted somewhere on the Internet. If its a picture on your harddrive, go to a site like imageshack or photobucket to upload it. If its on a website already then you're okay.
Wherever the picture is, copy the url. That's the address of the picture, shown up at the top next to where it says address, and starts with http:// Make sure you're viewing the picture itself, to be sure, make sure the url ends in an extention for a picture, eg .jpg, .png. If you've got several urls to hand (often get that if you upload to imageshack etc), then keep trying til you get the right one.
Feel free to post if you still need some help.