Upload & Sell: Off
No, the photo must exist somewhere else on the internet, somewhere directly reachable via a web server. Then you need an http:// link to it. For example, I have a domain where I put mine, and I can link directly from there. Many people here use Flickr, and there is some way to link from there too.
Hmm, that's exactly what I do from photobucket, obviously they output very small, perhaps I should link with flickr instead.
EDIT: OK that's sorted, there was an output uploader setting in my personal details on photobucket set to 1024px, I've changed it to 1280px for future uploads.