To change the icon for an application, go to the application settings.

An ever-growing icon set is provided with RxLauncher. They’re from RxUI, released under the Creative Commons Attribution, Share-Alike, Non-Commercial license, so you can get and modify them at will.

You can find the one you need by using the search field at the bottom.

Searching for Car will show car and cards icons.

You can also use your own icons, not necessarily from this icon set, without even having to install any third-party icon pack. Just select any image file from your phone by browsing from the application settings. You can use any kind of image format, especially .jpg, .svg, .png