Others will probably be more helpful, but I think if you go to www.photobucket.com you can upload and then link a photo from there.
You need an online site that will host your image so that you can copy the url link back into the profile or forum settings. Before uploading your image you may need to edit it so the dimensions are in ratio with the image size specified on the settings page. Otherwise it will show up distorted from the actual image.
I don't personally use photobucket as my ISP provides customers with some online space for this purpose, but I think photobucket has been mentioned here and possibly other hosting-type sites as well.
hope this helps get you on the right track....
Another one for hosting pictures for you is
http://imageshack.us/
In some ways better inasmuch there is no registation or account required.
Photobucket is better for your photos that you want to keep returning to - or letting others see, ie like an album. That's what I use Photobucket for anyway.
Both do the job as mentioned though.