Certainly

Posting a picture inside a thread and inside your signature is exactly the same thing.

when you post an image, you use a code, called BBcode (bulletin board code) which displays the image.

The code looks like this: [img] [/img]

and wherever your image is located goes between those two tags

so lets take this image that I have in my signature for an example...it's located here:
http://www.nathanledet.com/sigs/sig8ballsatisfied.jpg

When I put that exact link between those to tags, it will render out like so:

the code looks like this:
[img]http://www.nathanledet.com/sigs/sig8ballsatisfied.jpg[/img ]

(note, that on [/img ], I have a space before the closing bracket ]. I put the space in so that the forum wouldn't render out the image...just take the space out and it will work)

Do you already have an image uploaded to the web? There is a photo gallery on the site, or you can use other sites, such as Photobucket.com