You can't delete directly. You can deactivate the file.
Look at the upload list and click the thumbnail of the photo, at the bottom of the page you will have the option to deactivate. Optionally you can specify a reason, I think it's good to write something as 'reason'...
But. I can't see the reason of portfolio cleaning by deleting/deactivating some images. Rarely will buy somebody your images simply because your portfolio is nice.