Thank you for writing.
Could you ensure that backgrounds of the images are transparent before to insert them into Command Bar Buttons. If images have some color for background there is no way Command Bar to show them with transparent background.
I tried to reproduce the issue locally with image with transparent background and Office2007BlackTheme and everything is working as expected on my side.
If you continue experiencing issues, please provide a sample project that demonstrates the undesired behavior or problematic image, so I can investigate it and help you with it.
I hope this helps.
the Telerik team