I know NSFL has been “taken” on urbandictionary to mean something more extreme than NSFW, but NSFW really tells you all you need to know in a short form. I propose NSFL to mean
Not Safe For Lunch
That is, it will kill your appetite and very likely make you psychosomatically nauseous. For example, if you are going to post something entituled “Vacation souvenir” and then include a picture of A TAPEWORM you add “(NSFL)” to indicate it will be disgusting in that way, without making people sick by reading the description.