Below is an example of a "summary" block in the "Carousel" Layout. Gallery blocks select existing Galleries (here stored in an unlinked gallery called "People Eating") and allow you a lot of flexibility about how they display and what items of secondary content also display with each image. You add image titel, caption, clickthrough URL, and source URL in the gallery iteself. Then, when you enter a summary block into a page you can select what of that information you want to display or hide in the Summary editing menu under Display.