I think I just found a bug with uploading photos here at 500px: It seems that the actual upload time is considered for determining the order in which photos appear on the "Fresh" page.
So if you upload a photo and do not immediately publish it (e.g. because you first edit the description and tags), it seems that it never appears on the first page of the "Fresh" photos.
Or am I mistaken?
EDIT: I just tried it out again, it seems I was mistaken.