When you add photos to an animal's record, you have the option to select one as the cover photo. This photo will be the primary photo displayed on the animal's record and also on your main animal list when you first log into Shelterluv.
You also have the option of selecting "Cover Photo" as a column in the Custom Entities Animals Report, which would populate a link to this photo in your report.
The cover photo does not determine which photo is displayed first within the Shelterluv iframe. That iframe will display photos in the same order that they were added.