Copying an image from the machine (copying a file or an image from an image editing/viewing software) and pasting it into an editable field in the browser is, generally, a feature of the browser. In HTML the images is not a binary file like on your machine, but an img element with an src attribute that has to point to, generally, an URL.
With that in mind, when pasting an image copied from the user's machine, Firefox converts it to a base64 string and sets that to the src attribute of the img tag it generates. This is something the browser itself does, so an editable field (or RadEditor) does not know the image originated from the user machine.
In Chrome, there is API that lets us find the image in the clipboard and by using the FileAPI of the browser, convert it to a base64 string and generate an img element, in a similar fashion to Firefox. And we do that.
IE, however, does not offer either of these facilities, so pasting an image from the user's machine is impossible there. I am attaching here a simple editable iframe you can use to test this.
Now, on pasting images copied from the page or an editable field (RadEditor) - it is, again, up to the browser to create the clipboard data and determine whether it will be an img HTML element. So, when pasting, the browser provides the content and we cannot influence it.
If this does not answer your question, please explain the issue you see in more detail (or even better - add a video that shows what you do with one version and not with the other) so we can answer your outstanding queries.
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