Neil - with photobucket (the one I have) you have 4 or 5 options as to the type of link to use. I don't have dropbox but I would guess that is the same. If it is then try different types of link as they wn't all work here.
Nick - with Dropbox you can only copy the link, there is no option (as far as I can see) to select the type. So I don't think it's going to be suitable for this forum. I'll install photobucket and have another go.