Rapid Resizer's a website, so there's no app to install. You may be able to place an icon on your desktop. At the least, you can bookmark the New Project for easy access.
In the Chrome web-browser:
- From this page.
- Click the ⋮ button near the top right.
- Click "More tools".
- Click "Create shortcut".
In the Safari web-browser, there's no easy way to do it. You can just click on the Bookmarks menu and add https://online.rapidresizer.com/start as a bookmark in your Favourites. Then you can easily access Rapid Resizer from the browser's Favorites bar.