If you're in Mozilla Firefox, right click the image itself and "copy image location". if you're using Internet Explorer, right click the image and choose Properties then highlight and copy the image URL.
From there, head up to the top of this forum and look for "User CP".
When in there look down the left for "Edit Signature". Once in there, simply click the little picture box and it should pop up a field to enter the image URL into, paste it there and then hit OK, then save it!