this is how I do it (there may be a more efficient way, I'm not sure)
Once I've published my polyvore design, I screenshot my screen (there should be a button in your keyboard that says "scrn" or "prnt scrn" or something along those lines) then I go into Paint and press "paste", that will paste your screenshot onto Paint and then you can crop it so that it's just your design. Then all you have to do is save it and then upload it to photobucket. Like I said, there may be a more efficient way that I'm not aware of, buy that's how I do it. Hope that helped.