The LargeImage property of RadRibbonButton is of type ImageSource, thus you cannot set it to a UIElement object.
To achieve the desired goal, you can use an image or set the path to the button's content as in the second snippet from your post. Another possible approach is to customize the template of RadButton.
In case you choose to replace the Path with an image, you should have in mind that RadRibbonButton displays the image applied to the LargeImage property only when the button is in its Large state. In the other states (Small and Medium), the image set to the SmallImage property is shown.
If you decide to stick to the Path, keep in mind that you should handle the resizing of the image according to your preferences.
Hope this is helpful.
Telerik by Progress
Do you need help with upgrading your WPF project? Try the Telerik API Analyzer and share your thoughts!