This problem causes the handles of the widget to be rendered at an incorrect position. The resizable widget should dispose or repaint itself, when the image is being dragged around the content. 

Possible workaround is to reselect the element on the dragend event of the editor's content. You can follow this example setup:

 <telerik:RadEditor runat="server" ID="RadEditor1" OnClientLoad="OnClientLoad">
</telerik:RadEditor>

<script type="text/javascript">
	function OnClientLoad(editor, args) {
		editor.attachEventHandler("ondragend", function (e) {
			var selElm = editor.getSelectedElement();
			if (selElm && selElm.tagName && selElm.tagName === "IMG") {
				editor.selectElement(selElm);
			}
		});
	}
</script>