If you want to keep the larger size one on Photobucket, upload it again to get a copy that you can resize. Then click on it and go to the "Edit Image" dropdown menu above the image. One option is resize. Just make sure that it fits within the 800x600 or 600x800 dimensions that the contest requires and save. If you uploaded a new version to preserve your old one, you then need to edit your post with the new image link.
I am seeing the one in your original as still too long in width. The height is fine. Try resizing that side to 800 and let Photobucket select the height. Be sure to have the "Keep Proportions" checked.