[eluser]WanWizard[/eluser]
Spam suggests it's a deliberate action, and therefore difficult to fight.
One step is to allow uploads only for authenticated users, and use a proper registration system (one that sends out verification emails). That will stop some, but not all (see this forum as an example). A next step is to block accounts and/or IP's. That will block the offender, but not it's neighbor.
Use a spam reporting system (like this forum has), so that legitimate users can inform the administrator if they see something. That reduces the work you have looking at all the uploaded images.
If you have a feeling they are automated uploads (p.e. because of a high upload rate), use a good captcha, and limit the upload rate to one image per x minutes, or x images per day.
Reviewing before publication, of screening after publication are also options, but might require quite a bit of time.
And finally you can take drastic measures and block entire cities or countries. For that, you need a GeoIP service. Depending on how granular you want to be, you can use Maxminds free country database, or use their paid service which can give you cities as well. There are some web based alternatives that you could use, but they all limit the amount of requests, and get very angry if you misuse them for validation...
