Sometimes image URLs can get lost when a product is deleted/re-added, or if images are changed/deleted/re-uploaded. It happens.
Fortunately, there is a simple process to fix the image in your listings:
1) Go into the product that is linked to your listing and find the image you want (at the bottom).
2) Right click on the image and select "Copy Image URL".
3) Go into your listing, and paste the URL into the description of your listing where the previous URL (the broken one) currently sits.
4) Republish the listing to submit the changes.