I had the same problem. I removed all initialisation code (beside the call to InitializeComponent) in the constructor but still had this problem. I could solve the issue by adding a DesignTimeAttributes.xmta file with the following contents:
<Class Name="OpenNETCF.Widows.Forms.Button2">
<DesktopCompatible>true</DesktopCompatible>
<DesignTimeVisible>true</DesignTimeVisible>
<Property Name="BackgroundImage">
<Category>Misc</Category>
<DefaultValue>
<Type>System.Drawing.Image, System.Drawing, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a</Type>
</DefaultValue>
<Description>The background image displayed for the control.</Description>
</Property>
</Class>