I've added the option to flag individual images as 18+ / mature.
You will now see this feature in the image upload section, as well as in the image edit section.
Additionally, I re-arranged these pages slightly, as some people were missing the new categories and keywords sections.
The "mass update" feature will also have the 18+ flag, so you will be able to update all of your images at once. This should be online either this afternoon or Monday morning.