Tip 17: Reconstructing Prompts
Simplest way is just asking the creator.
If you can't, Midjourney has an image-to-prompt feature. First enter /describe:
Click describe, then upload image and hit enter.
Midjourney will return possible prompts - click the 1,2,3,4 buttons under the image to directly send prompts to the bot for image generation. On the right are images generated from prompt #1 and #4. It seems decent for reverse engineering composition, subject, style of original images, but don't expect identical results: