Nov 21, 2013 12:56 AM|Happy Chen - MSFT|LINK
hi fur.attila ,
it seems that it’s a ToolboxBitmap issue, you can search the related information on the forums:
I would suggest you read the reference below:
How to: Provide a Toolbox Bitmap for a Control
you also can Refer to the links :
How to: Create a Date Picker Composite Control in ASP.NET (C#)
Hope it helps you.