We have observed this issue only in Lightswitch projects - the images are shown only if they are added to the visual tree on start-up. The problem does not occur when there is a RibbonUI, because the ribbon includes the images in XAML.
The workaround we can suggest to you for now is to declare the images in XAML. This means, you should have a line like this for all images which are used in the SelectionMiniToolBar and the ContextMenu in your .xaml file:
The images can be collapsed or with Height/Width set to 0.
Here is the list of the used images in theSelectionMiniToolBar and the ContextMenu (please note, you should add in the beginning the "/Telerik.Windows.Controls.RichTextBoxUI;component/Images/MSOffice/16" string, as a part of the path to the following resources):
I hope this helps.
All the best,
the Telerik team