Well on the custom standards you need Photoshop, Gimp or another program to create and edit the pictures and you'll need Packfilemanager to make the mod, when you have your picture in all the sizes and formats needed then open packfile manager, right click add from pack and go UI -> Flags and select a random flag, now you have the path, so then right click on "Flags" and click add from files and select your custom made pictures
Note: Make sure your files have the exact name of the ones you want to replace or it will not work