REQUIRED: An API key from steamgriddb.com
This is IconGrabber. A homebrew that one can probably best describe as an unofficial steamgriddb.com client for the Nintendo Switch. You can search for games and then preview and download icons that you want. After that you can apply any icon to any installed title of your switch and replace the original. In order to do that, you need to use sys-tweak though.
The Switch default icons are 256x256 but the tool will automatically resize anything to fit. This means you can use 512x512 or 1024x1024 icons (as long as the ratio is 1:1). Once you applied a custom icon to a game, you need to restart the console for it to take effect.
This tool also works for vertical icon themes.