This game is called "The mural" and according to the internet here is the definition of a mural:"a painting or other work of art executed directly on a wall", and we're going to recreate a digital "wall", where anyone can create their art in their own style. So, to make it easy for everyone, this mural will be exactly 500x500px, that way it fits the size of the forum., and 4 people will create the first row of the mural, and 4 people will create the second row which makes 8 people to participate.
Each person will create a 125x250px file with their own creation, but, to make each row look like every picture is a continuation of the last one, from that same 125x250px file you will have to cut the last part of it and create a new 26x250px file so the next person take that file and attach It to their own creation and continue with the game.

Confused? I bet you are, so, here is an example of a very old mural I was doing with my best friend (not finished), our mural was horizontal so it was getting quite large at some point. Here is the original image, and the first part of the mural, this was 450x400px.



So from this very same file, my friend created a smaller one (basically cut it), the size was 100x400px.



I was the next person to continue the mural so i took this smaller part, created a new 400x450px file, paste this new 100x400 px part on the new art, make it look like is a continuation (i did some blending here), and then cut another 100x400 px part so my friend could continue. Here is my part, the only thing missing was the new 100x400 part for her to work.