practice makes perfect but remember your creative writing piece'll need to flow naturally so it's just learning about how you would write a good piece. try to be creative when responding to your stimuli and draw off it eg. what happened before the photo was taken; would the photographer play some part in the description/story perhaps.
show, don't tell when describing but i wouldn't recommend following a skeleton or something, just be confident in your techniques. there should be exemplar answers for whatever exam board you're doing online- worth printing these out and see how they crafted their response.