I'd recommend to use just md5 or anything conceptually equivalent.
By renaming files by digest of it contents you are not only granting uniqueness (always cache images for as long as possible, and with content-based renaming, well, with proper one, you can cache images practically forever).
Also, not a big deal, but nevertheless it is not a pure hypothetical case when different users upload exactly same image. Just out of the box you'll have a small data storage optimization.
As for anything else proposed: as for me, I am a strong opponent of keeping any kind of auxiliary information in a file name. When I was much younger (and a bit slimmer :), I've been a Perl developer and had a dubious habit to store as much of auxiliary info in file name as common sense allowed me, since Perl string pattern features are awesome. And I've came up to conclusion that, talking of web development, it is always better choice to keep data associated with file separately from the file name.
Keep in mind that nowadays, when mobile interfaces are dominating, actual file name is a less important thing that it was 5, 10 years ago. But even if this will be crucial in context of your application, you can always involve some old school magic with with involving
Content-Disposition: attachment; filename="pretty_file_name.jpg" HTTP header, constructing any relevant file name you wish. Also, modern browsers are paving way to new HTML5 attribute, download. I don't believe that actually seeing "human readable" image name is a thing you should think about in majority of cases.
UPD: A modification can be made in order not to have too many files in one directory - just take first 3 letter and create dir.