The easiest thing to do is to look for a post where the poster has the OKW photo you like in their signature. Right click on the photo - a box should open with properties as one of the choices (at the bottom). click on properties and carefully copy the url (double click to high lite the whole thing and then ctrl-c to copy). It will look something like:
Now go to your user cp (top of the page) and go to the edit your signature. you can press the little icon that looks like a mountain range (insert image) annd then paste your image url into that box (ctrl-v). The result will look something like this (note the "[ img]" and "[/img ]" should not have spaces - just something I had to do to show the code better).