First you need a webhost (take a look at http://imagemagician.com/ for a free image host)
Once you have created your image, you will need to upload it to the webspace and then link it using img tags.
you will need to replace "url" with the location of your image.
lol that image you are trying to put as your signature is far to big for the Forums.. it needs to be 500 x 200 in size. I know this because mine used to be alittle on the big side.. but im corrected now
Where the // are above, remove the space between the last / and atlas. I put a space there so that you can be able to see the whole link of which your picture is so you need to remove the space when you put it under your sig like Jewelzz told you. For some reason, the forum shrinks the web address