I've been uploading some of my better photos to 500px, and thought I'd order my photos by preference, but I can't find any way to do that. Is it not possible?
Currently there is no option to reorganize the images on your public profile, only within your portfolio can you drag & drop images to the order you like. We will be introducing an option to re-arrange and categorize your work, it is coming very soon.
From their Support Center.