I don't think you can attach more than one image, but you can add it like you would embed a photo. You have to upload it to someplace like Flikr that allows you to share photos from their site. Then click on the img icon (square with mountain) and put in the url for the image.
NOTE- please try to keep your signature to about 200 pixels high. Large signatures cause too much scrolling especially for members with small monitors. The 200 pixels includes photos and text so resize images if necessary. I've attached a sample of what 200 pixels looks like. Click it it to expand it to full 200 pixels size.