#010 - Broken flag images
This is not an issue with the game per se, but with the links used for flags. If you're using a service like imgbb or imgur, make sure the link you are providing as the flag links directly to the image and not the page on that service that shows the image.
It's easy to figure it out by looking at the link: if there's a .png or .jpeg or other filetype in the end, it's the right link.