They're like one of those old video games where you control your character through a series of suggested actions to take, like 'Lift boulder' or 'Head East' normally in the fashion of a verb followed by a noun. You can find them among the internet nowadays, such as
MS Paint Adventures, Ruby Quest, and White Room to list a few. It works as such: A single artist or a small group of artists draw the first image, usually giving you a quick summary of the area the main character is in. Someone then puts up a suggestion and the artist(s) then create the next image, with some sort of text below the image to assist with describing the repercussions of the suggestion.
Here's an example:
> You are Ember Firethroat, an elementalist who commands fire. You awaken in your room to the sun's rays shining through your window. You stand up from your bed and examine the room about you, spotting the rather obvious window and door which leads out to the firepit. The walls of your small hut are decorated with trinkets and shiny objects which you've found throughout your training, each commemorating the small achievements you have made.
What will you do?
>Examine the Trinkets<
>You take a step closer to the wall of Trinkets, and reflect on your past. Each simple item brings a flash of a memory before your eyes as you smile, remembering the fun you had as a child and the ambition to become an elementalist. A specific Trinket, which is a Ruby shaped like a serpent, catches your eye as it reminds you of when you discovered that you were best with the element of fire.
>Take Ruby Trinket<
>You unhook the trinket from the wall and fashion a necklace from a stray piece of string, hanging it over your shoulders and about your neck, your thumb and forefinger rubbing against it as you feel a slight warmth from it's shape. Outside you can hear what sounds like a celebration starting.
And it would continue as such.
Simple: doesn't it seem that way? What's better is that most of these Post by Image Roleplays are open to suggestions from the public.
I myself have even started one. So check them out, as it's like a story that you yourself had put a hand into making, because it actually is YOU who helps create the story.