To delete or change a photo simply log back into the site and under the actions column click on the 1st of the 3 icons to edit your listing. Then
scroll down to the photo section and tick the "delete image box" next to the wrong photo. Then scroll down to the bottom of the page and click the "submit" button.

On submitting you will return to you control panel page again. Go to edit the same advert and once you have scrolled down to the image section you'll see that the previously deleted photo/photos have gone. Upload your new image in its empty space then scroll down the page and submit again.