The problem most probably stems from z-index values, i.e., the z-index of the TabStrip's tabs are with higher value than the one of the RadEditor dialogs.
In a plain scenario, without any custom CSS the tabs stay properly behind the modality of the Image Manager. Therefore, this might be are result of some custom CSS implemented in the used page. You can investigate the rule affecting the tabs via the browser's dev toolbar and remove it.
If you are unable to resolve the issue, you can try to isolate this into a simple project and provide it, so that I could examine it properly and find the origin of the problem.
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