You can only delete (not swap) a photo from a challenge within 5 minutes of submitting it.

After that, you will need to swap the image.

Top Photographer:

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.

Top Photo:

Hover over your photo in the 'Top Photo' ranking until a  "Trash Can" icon appears.

Pressing it and confirm by pressing the 'Remove' button.

*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!