Thank you for your interest. Unfortunately, this issue has not been addressed yet. Our team apologizes for the inconvenience.
Nevertheless, I managed to produce a workaround for this case. The only way to apply custom images to RadPanelBarCaptionButtons is to set their Image
property each time the RadPanelBarGroupElements have been expanded or collapsed. This will also allow you to customize the image size, location, and ImagePrimitiveAngle. For further information, please refer to the attached sample project.
I hope this is helpful. Our team apologizes once again. We will address this issue in the future releases. Please, let me know if the provided workaround is convenient for you.
the Telerik team