1. Remove html-code
First you have to remove the html-code of the image from the page. Go to the page of the geocache and click Edit in the block at the top right of the page
Now remove the html-code of the image from the description of the geocache.
Save the changes by ticking both check boxes at the bottom of the page and click the Submit Changes button.
2. Remove the image from the list at the bottom of the page
The image is now still visible at the bottom of the page. Click next to the image on Edit.
Now click the red bin next to Delete Image and confirm this by clicking Yes on the next page. The image is now removed from the list.