Hi Tim Duquette,
We are aware of this behavior - it is caused by the fact that the images you refer to are set through the theme mechanism. However some of the themes have those images serialized inside the XML file (through Base64 encoding) thus having always access to the images. I believe those are the themes that render correctly. Opposite to that some of the new themes do not have the images serialized inside of them - when this is the case, the referenced image is searched inside an ImageList associated with the themed control which I believe is not set in your case. So the theme searches for an image, but because there is no associated image list, the images is considered invalid and no image is rendered (we do not have default images for the buttons). If you drop an ImageList component on your form, load it with the proper images and associate it with the ImageList property of the respective RadControl instance.
We are considering now how to generalize and make simpler the mechanism that references the used images inside a theme.