Are you trying to upload a picture directly from your computer? Then no, you can't do that. You can have your signature on a site such as Photobucket, and past the link into the field. Remember signature sizes are limited to 600 x 150 pixels (without any text above or below), and a file size no more than 30K.
I had uploaded it to my photobucket account before trying to put it onto the page, which is why I'm so confused about it. But i'm pretty sure it's too big though, so I'll have to fix that anyway. thanks.