The problem is due to some global
CSS style for example for TABLE, TR, TD, UL, LI A elements that is corrupting the design of RadEditor's toolbar. My suggestion is to inspect your stylesheet for such classes with background or background-image properties and remove them. If you unable to solve the issue, please send a live URL to the problematic page or a simple working project that demonstrates the issue. I will examine it, find what is causing the problem and provide a solution.
You can also try the provided solution in this KB article: Overriding Global CSS Styles Inherited by RadEditor
the Telerik team