Class Library Toolbox Icon

Step 1: 

Create bmp file (16x16) and save as like below.

   ProjectNameSpace.ControlName.bmp

  e.g. Project Name : CustomGrid
        Control Name : MyGridControl

Bmp File Name :  CustomGrid.MyGridControl.bmp

Step 2: Add in Project and Change Build Action Properties to "Embedded Resources".

Stpe 3: Add ToolboxBitmap line in your class file as per screenshot.

Step 4 : Compile your Project as "Class Library".

Step 5:  Add this control in Different Solution Project Toolbox using choose items and icon will display like below ( In Same Project icon can not display).

Download Complete Source : CustomGrid.Zip