The Image and Thumbnail types can be used to capture and display images in the app, and the Video type can be used to display video (video capture is currently not supported). These types should be used when image capture is needed or you want to display URL or file name values that are stored within your table data. If you don't already have a column reserved for your image or video, you'll need to add a new column first and change the column type.
You may also want to show a particular image or video within a form view whenever users are creating or editing a record, but without including it in the data as part of the record. In that case you should use the Show column type. In the Category field of the Show column choose Image or Video, then place an image link or video embed link in the Content field. Note that the link must be a public URL for the media itself, not to a web page that displays it (an image URL will typically end with .png or .jpg, for example). This is a common problem when trying to use images from Dropbox or Google Drive.
You can control whether Show columns appear in other view types besides forms when you configure the view settings.