Right, I'm well krap at instructions... first you need to host the image - I uploaded it to another forum I use as a post edit
Then go to your profile and select "edit personal picture) (I think it says)
Click on the IMG button at the top,
open another browser to the host of the image you want, right click the properties of that image and "copy" the url...
come back to your profile on here and add it to the box that opened when you clicked on IMG thats it I think but I'm sure someone else can describe it better cos I'm a dyslexic old bloke :*)