Full-bleed images: images that go from one edge of the window to the other; of no consequence on a small screen, but on a 27" monitor you can have a gorgeous view of the Alpine hills that someone shot last winter.

Alt-text: text that describes what is in the associated graphic, e.g., "Gorgeous view of the Alpine hills, shot last winter"; VERY useful for sight-impaired users, especially those with screen readers.

Captions: yup, the text beneath an image.

Image-grids: yup, what you get when you select two or more images to insert at once into your story.

Hope that helps, Josie!