You can only delete a photo from a challenge within 5 minutes of submitting it! - (updated 01/17/18)

After that, you will need to swap the image.

To delete a photo, hover over it with your mouse in the 'Top Photographer' ranking (the default when you go to any challenge).

A "Trash Can" icon will then appear and pressing on it will open a 'remove' button.

Press the button and the photo will be removed.

*Note*: You can still delete the photo from your profile at any time.

           Deleting a photo from your profile does not remove it from the challenge!