Such an issue can happen if you are setting very large
images as the ImageUrl of the items such as RadRibbonBar button. Check
the attached image radribbonbar1.jpg. I suggest you choose small images
so that it fit properly in the dedicated space.
Another suggestion is
you can increase the dimension of the image placeholder by overriding
the CSS as follows. Check the attached image radribbonbar1.jpg
.RadRibbonBar .rrbLargeButton .rrbImagePlaceholder