Currently there is no way to export theme images as Bitmap (or any other image type).

Resolution: In Q2 2014 SP1 we introduced feature to export images from Visual Style Builder. The issue is the same like ADD. Visual Style Builder - add ability to export base64 strings to images (export image option in the context menu of all Image items). You can follow the link: http://tpdogfood.telerik.com/view#item/111008