I'll be happy to help you out with this. There is a way to add a caption to every photo. Just follow the steps below.
1. Log into SiteBuilder and open the page containing the photo gallery.
2. Clicking on the photo gallery element will cause the photo gallery editing functions to appear in the properties editor on the right hand side of the screen.
3. Click the image name from the image list for which you want to add the caption.
4. Add the caption in the box directly below the image list.