SplashSizer is a tool for creating icons and splash screens in any sizes (it's made for GameMaker: Studio, but you should be able to use for any other application). The tool contains configure files for most of the platforms such as.: Android, iOS, Windows UWP, etc. But also you can create your custom configure files so you can use it whenever you want even for other purposes. So finally you don't need to resize the images by your hands, because with this tool you have it done within a few clicks. It also supports custom file names so you don't have to load every generated file into the GameMaker one by one, but you can just simply copy the generated files into the project folder, so it's super easy and super fast.
Forum link: https://forum.yoyogames.com/index.php?threads/splash-sizer.21550/
If you have any suggestion how to improve the tool, please let us know via e-mail. Thanks.