Below is a sample image gallery. If you hover over it, you'll see an option to edit. You can add or delete images, re-arrange the order or in the upper right, you'll see "Content | Design" options. If you click on design, you can switch between slideshows, stacked, grid, etc.
Here are different elements. Click and drag them around the page, everything will snap to a grid.
When you see hover over them, you'll see a gray circle-ish shape pop up, that's how you add new content (text, images, forms, links, audio files, etc.)