Gotham Knights' graffiti is one of the few collectibles you can hunt down in the game, rewarding you with some XP and crafting materials if you locate all 12. That said, some of this street art is actually quite troublesome to find. You'd think giant murals would stand out, but Gotham is a bit of a multi-leveled maze as far as cities go.
Once you locate a mural, you need to scan it in order to get progress, and when you do it'll tell you a lil tidbit about the history of Gotham and what's being shown. If you hover your map cursor over a region, you can also see whether there are any murals left to collect by looking at the spray can symbol in the bottom right corner, which will have a green tick if you've found them all.
In this Gotham Knights graffiti guide, I'll give you the locations for each mural, directions to get there, and you can also see what each mural looks like if you cycle through the galleries below. I've arranged these in the most convenient order of collection starting from your home base at the Belfry.
Green Gotham - Financial District
This mural can be found exactly west of the Belfry on the side of a building with a water tower, looking directly onto the train tracks.
Stolen Gotham - The Cauldron
This one is on the side of a small building and looks directly onto a fenced parking area. Since it's on the upper level of The Cauldron you can also see it while travelling south on its west side.
Faces of Gotham - The Cauldron
The easiest way to find this one is to travel south on the central bridge through The Cauldron, then drop down to the left side into the lower level when you see the Cobblepot Steel billboard. Turn around and the mural should be right there against a wall.
Simpler Times - Old Gotham
This one is easy to find since it's on the main street junction just to the east of the Gotham City Cathedral.
Gotham Piers - Tricorner Island
Heading along the bridge to Tricorner Island, take the first off-ramp on the right, keep going straight and up a small flight of stairs, and you'll see the mural on the side of a house to your left.
Lyceum Mural - Bowery
As you're about to head onto the bridge between Bowery and Bristol, you can see this mural on a building just to the right.
Origin of Evil - Otisburg
Just to the east of Wayne Tower, this one is against a wall underneath a railway line that crosses the road.
Born This Way - West End
This one is quite easy to miss, but if you head west down Croydon Avenue past the Gotham City Labor Union and around the corner, you'll see it on the right side, slightly raised up from street level.
Our Friend Joel - West End
Heading across Madison Street Bridge from the Financial District into the West End, you'll spot this mural to the right on the building behind the gas station, just as you pass Pamela's Cafe Diner.
GCU - Gotham Heights
Travel across Aparo Bridge from the West End into Gotham Heights, and immediately turn right down the small street as you arrive. The mural is on the side of a building to your left.
The March of Crabs - Robinson Park
This one is a little trickier to find since it's on the border between Gotham Heights and Robinson Park. Head to the statue of the globe with tentacles in the western side of the park, then head west under Chambers Street to find this mural against a wall to the left.
See Us - Bristol
You'll find this one next to the basketball court surrounded by the big residential buildings on the south side of Bristol.