Web sites with large pictures often display NSF56K meaning "Not Safe/Suitable for 56K". Contrast with NSFW, "Not Safe/Suitable for work", used to mark questionable content should your boss be shoulder surfing.
Those jpgs of Veronica Zemanova are 800K each! Definitely NSF56K
Not Safe For 56K: containing more data than is sane to download via 56k modem. From "NSFW", relating to items that should only be looked at in a work environment, especially if the office is open-plan.
Pages containing many high res pictures are often NSF56K.