We have recently logged a similar bug in the RadRibbonWindow in our PITS. Unfortunately, we cannot promise an estimated time frame when this will be resolved.
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One thing you can try in order to improve the way images are visualized in RadRichTextBoxRibbonUI is to enable layout rounding.
This can be done only in WPF 4.0 by setting the UseLayoutRounding property of RadRichTextBoxRibbonUI to true, as described in this MSDN article.
I hope this helps.