Unfortunately, we can't delete covers posted to the main database. If we could, we'd go on a deleting rampage. Back in 2006, when the site first started, there were no standards and it was like that for years. Luckily, we've replaced the larger volume of covers from that period. However, hundreds still remain.
Remake the cover, and submit it. And send me a personal message that it's intended to replace the old one on the site.