If a picture used to display correctly and no longer does so, the most common reason is that the picture is stored externally on your hard disk, and owing to some change on your computer, the stored 'links' to this external file (called 'external file links') have stopped working and become 'broken'. There is no need to delete the picture and re-insert. You can check all external file links, and easily manage and fix any broken ones, using the Work with External File Links Dialog. To open this dialog, click on on the Tools menu.
It is well worth taking the time to understand how this dialog works. It may appear a little complicated but at first, but it is actually much simpler than it may seem; and once you have understood how to use it, you may find that a single minor change in one place can fix any number of bad links at a stroke and potentially save you a great deal of time and effort as compared to any other technique you might care to use to repair bad links (see Work with External File Links Dialog).
Tip: There are two useful links at the bottom of the Media Record tab in the Media Window Display Pane (when viewing a single picture), labelled 'File' and 'Location' respectively. Clicking on the first link opens the media file (the file on your hard disk containing the media item) in an external viewer, and is the equivalent of clicking on the Open Editor/Player button on the toolbar. Clicking on the second link, the Location link, opens File Explorer to display the media file in its location on your hard disk. If the media file cannot be found on the hard disk, the first link will display in red, prefixed by 'File not found - '. In that case, clicking on the link will open a File Not Found dialog box, with options for locating the missing media file - which includes letting Family Historian search for the file for you.