In order to change the photos that will be the cover of your ad, just follow the steps below:

1 – Enter your private area in the “Your ads” section.

2 – Click the “Modify your ad” button. ( On the page says “EDIT”) 

3 – In the “Personal Images” area click on the top of the photo you want to put as the cover of your ad in the “Set as preview” part.

4 – Click on the “Continue” button.

Done!! You have the cover photo you have selected.