What does "sup /b/" mean

I see this all over the internet written down and drawn on pictures of things. What does this mean

what is up, brother?

Far as I can tell–and it’s a tough thing to search for–it appears to have something to do with 4Chan’s infamous /b/ board. What, specifically, it means I have no clue.

‘/b/’ is an image forum on the website 4chan. See the wiki for more details, though I warn you against visiting the actual website.
So “sup /b/” is just the members of the forum communicating, saying hello (sup)

I was going to say the same thing regarding 4chan. Sometimes these things really don’t mean anything.

To verify a photo is real instead of a photo that was taken from another site, they person in the photo will often write “sup /b/” (“What’s up /b/?”) on something, such as their hand, before taking the photo.

This. Usually in the context of naked pictures of oneself, and a date is often added as well.

You asked.

Valete,
Vox Imperatoris

Huh?

Alright, someone posts a naked picture on 4chan. To verify that they are really that person and not just taking a picture from an “adult” site, they write “sup, /b/” and the date and time on their hand.

Valete,
Vox Imperatoris

/b/ is a seething cesspool, but at the same time, a pretty influential force in Internet culture. You know all those memes that circle around over and over? Those have to come from somewhere, and typically /b/ is a major source of them. See, for example, Chocolate Rain, or Rickrolling.

/b/ is 4chan’s ‘random’ board. Here is all you likely need to know about 4chan. There are other chans, but 4chan is the biggest one. ‘Chans’ are image boards. For example 4chan has image boards related to various flavors of porn but also on various other subjects, such as the paranormal.

I won’t link directly as /b/ can have very NSFW content, and 4chan in general can. However, all you need to do is go to 4chan’s site and bring up the “Random” category. That’s /b/.