As in question, how can we assign an icon here?

Regards
That's the Launcher.
When you run a program, it's icon appears in the Launcher. If you right click on the icon in the launcher, you can select to lock it to the Launcher so you can run it quickly in future.
If you have lots of them in the Launcher, you can resize the Launcher icons as required by selecting Appearance in System Settingsand moving the slider as required.
