Hi Beverle,
The instructiuons mentioned above by "ninth_wave" will work for adding images to the description your listing, and for updating any other information within the description section.
However, since the gallery image is not entered in the description section, an additional process is required to update your gallery image. In order to change the gallery image in an active listing, you can revise the item via the following process:
1) Visit the item on eBay.
3) Right click on the image you'd like to make your gallery image and click "Properties"
4) Copy the image URL...it begins with "http://..."
5) Click the "Revise your Item" link on the auction.
6) Click the “Add Pictures” button.
7) Delete the text in the “Gallery Picture URL” field and replace it with the copied URL.
7) Click “Insert Pictures” and then save your changes.
-Mike