On the bar at the top of the page click on "Showroom".
Just above the pics that appear, there is a bar and on the far right click "Upload Photos".
Now click on the browse button and select the pic/pics you want to upload to the site and then click the "Upload/Submit" button.
Also, before you click "Upload/Submit" you can title your pic, and/or enter a brief description.
Once the upload is complete, simply click the process button.
Now you have a pic in the showroom, from there you can add it to your sig or just post it in a thread.
Easiest way is to copy to clipboard the UBBCode and then all you need to do is right click and paste it wherever you want.