I think to do that ya gotta have a URL and it would need to be of a suitable size. If I feel they are too large I delete them.
You just insert the URL with proper code into signature line just like you would text. To get the proper code you can act like you're making a post, put the URL down and highlight it. Then click the IMAGE ICON
and it will put the proper code there. Then just copy that and paste it to the signature line.
There are limits to how much can be in a signature line both text and image. So long as you don't exceed it all will be fine. You are really close you have only 59 more characters to reach the max of 600.