Gantry provides the ability for you to create your own custom presets based on any parameter in the template administrator.
Go to Extensions → Template Manager → rt_corvus
Configure the Settings
Click Save → Save Preset
Follow the Preset Saver procedure
You can edit the prebuilt presets in the gantry.config.php file, or use the User Interface method outlined above.
When you create a new custom preset, there is a default image used in the preset chooser to represent this. You can create your own thumbnail for each style by creating a png file that is the 'short' name of the preset with the dimensions of 180px x 100px.