If you know the code to add a picture in a post, you know how to use an image for the signature, just use the same BB code also for that.
This is the code to use to let diplay any picture. Is important that the used URL is a direct link to a picture, like this:
https://www.mm2x.com/images/logo.jpg; PHP or other kind of codes that brings to the visualization of an image can't work.
The important thing to care are the signature character limits and the image sizes, that haven't to be too big to disturb the posts reading and enlarge the page width.