You need to edit the config.txt and add in a thumbnails container.
Code:
thumbnails
{
0
{
image "screenshot.jpg"
width 240
height 180
}
}
Create an image at 240 x 180 pixels.
"screenshot.jpg" is your image name, you can use whatever you want.
The image needs to be in the asset folder.
Once you've made the change, be sure submit the asset.
Hope this helps.