Or you could select the game in Launchbox click RMB and select Edit Game or use CTRL+E to open the dialog where you can edit all the game's details, including images.
If you then click on the image at the bottom right of the screen with RMB you can use 'Add Image' to add an image and 'Image Type' to set which type of image it is (box shot, game title screen, etc).
You can also drag and drop images onto that screen to add them, just don't forget to set the 'Image Type'.