You don't need to add the country image to the SharePoint list; instead, you can have a collection locally in your app that correlates the countries with their respective images. For example, if you have this in your App's OnStart property:
ClearCollect(
CountryFlags;
{ Name: "France", Flag: "https://upload.wikimedia.org/wikipedia/en/thumb/c/c3/Flag_of_France.svg/1280px-Flag_of_France.svg.png" },
{ Name: "Azerbaijan", Flag: "https://upload.wikimedia.org/wikipedia/commons/thumb/7/74/Flag_of_Azerbaijan_%283-2%29.svg/1280px-Flag_of_Azerbaijan_%283-2%29.svg.png" },
{ Name: "Bulgaria", Flag: "https://upload.wikimedia.org/wikipedia/commons/thumb/9/9a/Flag_of_Bulgaria.svg/1920px-Flag_of_Bulgaria.svg.png" },
{ Name: "Germany", Flag: "https://upload.wikimedia.org/wikipedia/en/thumb/b/ba/Flag_of_Germany.svg/1280px-Flag_of_Germany.svg.png" })
You can have in your gallery's image an expression that performs a LookUp in that collection, something like
LookUp(CountryFlags, Name = ThisItem.Country, Flag)
If you need to use this "flag lookup" in other apps you can also have it as an Excel table (that is imported statically to the app) or even in a separate SharePoint list.
The attached app shows an example of this technique. To open it, save it locally, then go to https://create.powerapps.com, select Open, Browse, and find the file that you previously saved.
Hope this helps!
