The top and the bottom parts of the ImageEditorDialog are invisible because they have fixed Width and Height properties and your resolution is smaller that their size. So, the way to fit the dialog into your screen is to set its WindowState property to "Maximize".
You can do this implicitly - adding references to NoXaml binaries and using external themes. Here you can read our detailed article how to do that.
After you have merged the necessary resource dictionaries in the App.xaml file, please add the following namespace:
Then you should add the following piece of code into the same ResourceDictionary:
Please, excuse us for the inconvenience.
Hope this helps.
the Telerik team