If you're trying to add a picture to the signature area, currently we don't have a way to upload images into signature space, just yet.
However, it is possible to link an image through a media/photo site such as ImageShack.
Normally a forum code would be provided once an image is uploaded:
You would then take this forum code and copy/paste it into your signature section like this:
After this, all you would need to do is delete the second string starting with "Uploaded with" as that is just a small blurb that would show where the image is hosted from.
You should get the image like shown in my signature below once you have saved.
Just in case, if this was in reference to Avatars as well, when you go under Edit Profile, make sure the Avatar option is checked to yes. You can then upload an image which will display under your username:
Hope this helps!
Ooh, what does that button do?