Thanks for reporting this problem. We fixed it and this will be working in the Service Pack 1 which we're about to release next week. This issue was caused due to a problem with the serialization of the StartButtonImage property.
Regardless of this you can set the property manually either from the .cs or the designer.cs file of your form.
e.g.this.ribbonBar1.StartButtonImage = global::MyNameSpace.Properties.Resources.MyStartImage;
I hope this information helps.