How do I stop image spam from being uploaded to my (future) site?

Question

I have in mind an idea for a generally accessible site that needs to allow images to be uploaded. But I'm stymied on how to prevent image spam: porn, ads in image form, etc.

Assumptions:

• I'm assuming that the spammers are clever, even human.
• I'm skeptical of the efficacy of image analysis software.
• I do not have the resources to approve all uploads manually.
• I am willing to spend money on the solution -- within reason.
• This site will be location-aware, if that helps.

How does Flickr do it or imgur? Or do they?

Answer 1

Flickr actually pays people (not all that much) to look through uploaded images for pr0n and other violations. There really is no silver bullet here. They have a whole queueing and payment system set up for reviewing uploads, and they even have multiple people review images to protect against those who let questionable content slide.

So yeah, you're going to have to pay some money on an ongoing basis to make this really work the way you want to. Failing that, IMO the best you can do is make people sign up for an account so that there's possibly a modicum of accountability when someone does raise the red flag after the fact.

Answer 2

There is no surefire method of sanitizing user-uploaded images. Even image analysis software will miss a small percentage of images and have false positives.

Flickr has community guidelines that say that you must flag your content appropriately, ie. if you're uploading adult images, you need to flag them as restricted. Otherwise, they will take action against your account.

Often you'll see compromises in cases where it's not feasible to moderate all user uploads:

• you might give your users free reign to upload content, but provide "flag" functionality so that other users can notify admins of bad images.
• provide "cooldown" periods for new users to prevent scripted bots from logging in and uploading pictures immediately
• flag problem accounts for manual image verification
• IP black-listing or grey-listing for problem IPs

You could even combine approaches, such as having image analysis software trigger a "requires moderator attention" flag. Unfortunately, there's no surefire method.