When you click on the 'add photo' link a window appears where you normally choose which pics to upload to ebay. If you look at the top of this window you should see a tab marked 'Self hosted' click on this and then enter the addy of the picture you want to upload. On photobucket it's the one marked 'direct link' or something similar.
The one problem is that ebay seems to have restricted you to one photo.
I think you can add more but it requires you to edit the HTML code. They've also put up the price of additional photos when you use their service. First one is free as usual but they now charge 36p per additional photo!