See my comment above - User configurable filtering based on also tagging by users - if the author sets it then it is locked that way, but other users can vote it nsfw and users set thresholds and image hiding functions.
Designating successors for accounts and potentially having child accounts are two definite needs, this would be a good step. Would be censors have to invest a lot of money to distort the threshold below the average user's definition.
Leaving this to flagging is worse than an optional filter. It is a natural extension of muting and low rep hiding. Sure if it is some nasty rotten.com style image, flag it, but a boob would not be nearly so nsfw, and simply being given a subjective rating for filter users... disable it and little is affected for those who don't mind seeing a little sometimes. Rankings allow users to set a threshold they define in the interface.
I'm actually not a fan at all of tagging content to suit the prudes and the parents. If a site is going to brand itself as an uncensored forum, then people get what they deserve if they browse it in a workplace that finds the content offensive. Neither is the site responsible for anyone's kids. If a parent allows their children to have unrestricted access to the internet, that is on the parents. It's not on me to shield their childrens' eyes from the things adults do.
I mostly agree, but I liked your checkbox solution. After that it is up to everyone to handle it. :)