1) Find the image you want to use
2) Upload it to a picture hosting site (e.g. Imgur)
3) After it's done uploading, right click and click "View image"
4) Right click again and click "Copy image url" (It can be different for other people)
5) Go onto your forum profile in edit mode
4) Go down to signature and use the code [.img] [./img] (Remove the full stops otherwise it wont work)
5) And paste the image url in between the image coding (Where the brackets close then open again, So in the middle)
6) And that should be it, You can preview it by clicking "Summary" on your profile page.
Hope this helped.
Oh, Well if you need any help either PM me or just post on this lil thread. c: