to get the right resolution for the photoshop contest (800x450) you should resize it to ( 450/720 = 0.625 ) 62,5%.
Accordiing to photobucket's resizing tool, 450/720 would mess up the photo's proportions... since adjusting the lenght to 720, the width goes to 405. This of course begs a question... which would be more correct? The height at 450 or the length at 720?