Images may be added to any item using the Rich Text Editor. to add images, first place the rich text field into edit mode by clicking on the field. Then click the the image icon from the formatting bar. An image modal is then displayed with a URL input.
There are 3 ways to add images using the URL input. You may enter a URL where the image is stored. That URL can be from anywhere you have access, such as a company server, dropbox account, or other storage location. If the item being edited has any attachments in the Property Pane pane, those will be options as well. Finally, you will also be able to select images stored in the project's Document app. So, you can create an image library within the project's Document app, and use that to manage images that will be displayed in items.