I will try to help you. I believe you are speaking of the image carousel on the main Gallery page:
Note the "Featured" tag on the image. If you choose to feature images, a manual process, then this carousel will scroll through ALL featured images. If you do NOT feature any images, the carousel will scroll through the last 5 uploaded (most recent) images.
For my purposes, when a member creates a personal album, I select one image in their album to feature. To feature an image, you open the image, Scroll down to the "Manage Image" drop down menu, select "Feature" to toggle on/off as a featured image.
Hope that helps you.....