Hi blueeyemom,
While you can select your gallery image first and then move on to images 1-24, I believe the best workflow is to add your images in the "Select an Image" boxes labeled 1-24 and then choose your header and gallery images. When you do it this way, you can just select the number of the image you would like to add from the drop-down menu labeled "eBay Header Image" instead of having to select the image from your library.
Also, if you are listing to the eBay US or eBay UK websites, the gallery image box will automatically be filled in with whatever image you select in the header image box. This has only been the case since ebay made the gallery image a free feature.
If you have any additional questions or concerns, please feel free to contact our customer support team using the web form on the following page of our site:
http://www.auctiva.com/help/requesthelp.aspx-Mike