A Dynamic Product Ad (DPA) shows your visitors the exact products they were viewing in your store along with other trending ones. For example, if your visitors viewed items A, B, and C on your site, the ad would show them items A, B, and C! Since this ad shows the items the viewer has expressed the most interest in, they are the items that the viewer would most likely purchase if they were to return to your site.
Since Dynamic Product Ads are unique to each visitors that sees them, the images they display cannot be edited. The preview is just an example of what the ad might look like to your visitors!
In addition, if your products are sold out, they will not appear in the carousel of ads.