If you have loaded photos to your listing and would like to change the order in which they appear, go to Add/Edit, select the desired listing from the drop down list and click Manage Photos.
At the top of each photo there is a little bar (where it says Primary slot, Slot: 2, etc.). When you put your mouse cursor in that bar, it will turn into a four headed arrow/cross:
When you have that arrow/cross, hold down your left mouse button to grab the photo. Now you may drag that photo and release it in another slot.
It can be a little tricky to rearrange photos. When you are dropping it in a new spot, make sure to drag it so that it is slightly to the left/in front of the photo that you want it to appear before.
For instance, if you wanted the selected photo to appear between photos 1 and 2, drag the selected photo so that it is on top of, but slightly to the left of photo #2. When you release the mouse button, it will drop the photo to slot #2 and push all photos back one slot.
Remember to click the Certify and Save button when you are done.