To re-arrange Unit Photos after you have already loaded them you will need to follow the Steps listed below.

Step 1) Cogwheel > Units > Units

Step 2) Once you have Selected the unit you want to change, select the Photos tab

Step 3) Once the Photo tab has been selected, you can see the box that appears below the photo. Changing this number will change the order in which the photos appear. Once you are happy with the order of the photos select Save.