Visual Verse offer an image as a prompt and invite writers to come up with a short story or poem in just one hour. (You start when you like and time yourself.) It's probably more of a personal challenge than a competition against others, although as they say they'll publish 'up to 100 writers over the course of the month' it seems reasonable to suppose that they'll select the best pieces.
This seems to me like a fun thing to do. Can you spare an hour to have a go? If that's too long, or you fancy a warm up, you can try this challenge – I'm giving you three minutes, starting from now, to write something prompted by this image. You can either post it below, or if you decide to keep working on it and submit it somewhere, just say that's what you're doing.