Rapid Resizer's a website, so there's no app to install, but if you use the Chrome web-browser you can place a shortcut on your Desktop:

1. Go to the Rapid Resizer's New Project page.
2. Click the ⋮ button near the top right.
3. Click Save and share.
4. Click Create Shortcut. (If you're on a Mac, Chrome will place the icon in a folder. Drag the icon from there to your Desktop.)

If you use a web-browser other than Chrome, you can bookmark Rapid Resizer to make it easy to return to:

1. Go to the Rapid Resizer's New Project page.
2. In your browser's Bookmarks menu, click Add Bookmark.
3. Save it to your Favorites or Bookmarks Bar so it will be in the row of bookmarks near the top of your web-browser.