How can I add a preview image (not an icon) to a .desktop file that will show up when I select the program in the Dash?

Before I start, let me just be clear, I am NOT asking how to add an icon. When I go to the Dash, and right click on an application, it says "No image available."

How do I add an image?

Here are the contents of my .desktop file under ~/.local/share/applications/.

[Desktop Entry]    
Name=StarMade    
Comment=A voxel-based space exploration game.  Build awesome ships and starbases!  Explore an infinite universe!    
Exec=/home/curtice/.steam/steam/steamapps/common/StarMade/starmade-launcher    
Icon=starmade    
Terminal=false    
Type=Application    
Categories=Game;Simulation;    
StartupNotify=false    
Keywords=sandbox;world;mining;crafting;blocks;multiplayer;roleplaying;voxel;space;battle;strategy;

P.S. I am running Unity 7.4.0 in Ubuntu 16.04.2