There should be a pencil icon underneath the piece, next to the heart. That'll take you to the edit page, and there is a "Delete" button at the bottom of that. If the pencil icon not there, you can get to the edit page by entering this URL:
http://pixeljoint.com/pixels/submit_icons.asp?id=113751
(You can get to the edit page for any of your pieces by replacing the number at the end with the ID of the piece, which you can get in its gallery page URL.)
Ah, sorry. I'm still new to the website, I'm having trouble finding how to remove this to move it over.
If you're still working on this, I recommend posting it in the WIP forum. The gallery is for finished work, WIPs aren't likely to be accepted, and people are less prone to giving detailed feedback since they treat submissions as finished.
nice!