When you first create a popup or image link gallery, you are given a set of options. The first is whether to place the gallery in the main content or sub content section. Next is whether to show the image title for the thumbnail. Then whether to show the image title in the popup (Popup gallery only). Split Gallery is optional (Popup gallery only). With this you can span a large gallery over multiple pages. Just enter the number of images you want on each page.
These options will be available as settings after the gallery is created.
The next section is where you add the image. Enter the image title. When Auto Thumb (Popup gallery only) is checked, a smaller version of the image is used for the thumbnail. If you're creating a link image gallery, there is a field for the link. Enter the full link path (eg. http://bigearthweb.com/ or https://bigearthweb.com/?act=view&page=Home). Finally upload an image (And thumbnail if Auto Thumb isn't selected) to the server.