I think I'm going to start doing these. I will show a picture that I have and the task is to write a story around it. I saw this on the side of a trail in the city while riding my bike.

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The genre can be whatever you want. Mystery, thriller, horror, fantasy, supernatural, etc. based on the image. Give it back story, perhaps. Be creative and have fun! 

There is no required length. Just write your story with however many words you feel comfortable.