A caution regarding image sizes
Each section with the option to upload an image (a "Browse" button) is designed so that images must meet a minimum image size to display correctly (or at all, in many cases).
If you click "Browse" and upload an image from your computer, you will see the minimum size displayed below the upload button. If you choose an image that doesn't meet the minimum, you will receive an error message.

If you navigate to the media library to browse for an existing image instead of uploading a new one, it is possible to choose one that does not meet the minimum size and you will not get the error message. If an image that does not meet the size requirement is selected in this way, it will appear on the back end (in editing view) but will not display on the front end (what site visitors see).
Minimum Sizes for Images
Article
Header Image: 1366 x 548 pixels
Thumbnail: 600 x 600 pixels
Basic Page
Intro Background Image: 1366 x 480 pixels
Book
Book Cover: 200 x 450 pixels
Event
Header Image: 1040 x 526 pixels
Thumbnail: 600 x 600 pixels
Image Card
Thumbnail: 600 x 600 pixels
Multipurpose
Introduction
Background Image: 1366 x 480 pixels
Main Content and Sidebar
Icon: 420 x 300 pixels
Video Spotlights with Articles
Image: 800 x 800 pixels
Dark Callout
Left Image: 800 x 800 pixels
Spotlight - Video
Image: 800 x 800 pixels
Spotlight - Static Image
Image: 800 x 800 pixels
Spotlight - Fullscreen Video
Image: 1366 x 800 pixels
Fullscreen Slideshow
Image: 1366 x 800 pixels
Timeline
Header Image: 1366 x 480 pixels
Static Slideshow Image: 600 x 600 pixels
Slide Background Image: 600 x 600 pixels
Gallery
Image: 600 x 600 pixels
Person
Headshot: 684 x 684 pixels
Testimonial
Testimonial Image: 680 x 680 pixels