Pasting of data through the clipboard greatly depends from the source and the target platform and environment.
The text data fetched by any application is text and can pasted as simple text, additional formatting and styling could be provided through the clipboard data and properly translated to XML, HTML or Word content, but in the RadEditor control this is a matter that is also precessed by the browser.
When it comes to images, in browser copying and pasting of images is achieved simply by transferring the IMG element itself just like a text data/HTML element. Although when a file is pasted in browser a binary data exist in the clipboard event and the conversion from binary image to a base 64 image (supported in the browser) is a logic that is natively supported in the browser and without it the RadEditor could not handle such data.
I hope this information clarifies why the pasting is inconsistent when using desktop and web application environments.
Check out the Telerik Platform - the only platform that combines a rich set of UI tools with powerful cloud services to develop web, hybrid and native mobile apps.