If you click your username in the top right hand corner it will bring up a menu, there is one in that list called signature. That will then let you edit whatever you want to appear at the bottom of each of your posts.
Strange, did you put the whole image address in mate? Click on the link below and copy the whole address from the address bar and put that between the img tags.
Yep, sorry mate, I just checked and that doesn't work in sigs. Try this:
1) Go to your sig box and click on the insert image icon (next to the smiley face)
2) Type in the image url (as above)
3) Once your image shows up in the box, click to the right of it and drag left, to highlight it.
4) Click the link icon (the little chain 7th to the left) and type in your website url. Click insert.
5) Click Save Changes.
Sorry for the bad info mate, hopefully this works for you.