Where are the icons in the ALT-TAB switcher loaded from? How can I edit them?

On my Linux Mint 17.2 Cinnamon system, ALT+TAB switcher, in the default settings (icon and thumbnail pop up in the switcher) shows a pixelated icon for one of my applications. I do not know where the icons are loaded from, and as a result I do not know how to fix this. So where are the ALT+TAB icons loaded from into memory for the ALT+TAB switcher to use, and how can I resolve this issue of a low-resolution icon being loaded?