I'm not sure it's possible as default. I assume you want to do this as some images break your layout? If so, you could probably solve that using some CSS.
In the templates, wrap each user's signature in a div (or p, or whatever suits your purposes best) with the class signature. Then in the CSS:
.signature img {
max-width: 600px;}
Set it to whatever is right for you. Of course, it will still load the full file size, but it won't display more than 600px of width.