Run the sample project and have a look at the gif file for illustrating the undesired behavior.

The issue is reproducible only when AllowAero = false.

Workaround:

        public RadRibbonForm1()
        {
            this.AllowAero = false;
            InitializeComponent();

            this.RootElement.Shape = new RoundRectShape(0);
        }

We are designing our application at design time. All changes to the button elements are done in the designer. However, the ImagePrimitive.MinSize property gets reset after a new resource image is added.

Please refer to the attached gif file.