SharePoint has a large library of app icon images on it’s servers, these are the icons that are used for the App tiles in Site Contents for example. You can use these icons to enhance web or app part appearance, promoted links, or for general use. These images are white with a transparent background so you will need to add a background color.
Here’s a screenshot of the app icon images with their file names:
The path to these images are: /_layouts/15/images/
Then just add the file name to the end of the path: /_layouts/15/images/LTAL.png
To see how these icons can be used for web/app titles click here.