Try going to the little dropdown by your name at the top of the site, edit your signature and click the little tree? Is that what you're wanting?
There should be a button in the tool bar that shows a picture of a tree. Click it and insert your image.
That is if you want a picture in your actual signature. If you want your profile picture go to "Profile" , then click "Avatar" , then click "Choose File" , finally choose a profile picture.
Well I don't know how you could put it into your signature. You could try copy and paste, but it most likely won't work. Try uploading it to imgur, then copy and paste the url and upload it in the "tree" box.
Then you need to do exactly what I said - upload the image to imgur.com, and then copy paste the image itself into your signature.