Hello queasylistening,
If your listing has a corresponding Saved Listing, you will have the option to Edit and Relist from the closed listings page of your account. The image selection section of the listing creation page is current set in such a way that if you remove the Header/Gallery images from their locations, the system automatically moves the next image in line (generally the #2 image) into that location, and will proceed through the list of already selected images. Once you have passed the existing images the pulldown above the Header and Gallery images becomes active again and you can use the Select Custom option to use any other image.
As for rearranging the order of images in your listing, unfortunately, there is no capacity to change the order once the images are assigned to the listing without deleting and re-adding them in the order you wish.
- Craig