The following code snippet can be used to prevent this behavior:

<telerik:RadEditor ID="RadEditor1" runat="server">
</telerik:RadEditor>
 
<script type="text/javascript">
    (function (Editor) {
        var setCursor = Editor.Selection.prototype._setCursor;
        if (setCursor) {
            Editor.Selection.prototype._setCursor = function (node, range) {
                return Editor.Utils.isTag(node, "img") ?
                    this._setCursorAfterNode(node, range) : setCursor.call(this, node, range);
            };
        }
    })(Telerik.Web.UI.Editor);
</script>