You should create a thumbnail for every color theme. The width of a thumbnail should be 780px. The name of a thumbnail file should be
themename is a name of the theme directory. Place the created thumbnail file into the root directory of a template.
Other preview images will be converted automatically. But to increase their quality, you can make it yourself. These images should be placed into the root directory of a template.
The names of these images and their maximal sizes are the following: