Let me query to the God of the presumption.
But in your code:
<DesignerControlLib:DesignerCanvas Width="600" Height="600" removed="#f5f5f5" Margin="10" >
<DesignerControlLib:DesignerItem Width="100">
<Image Source="/Kartvizitci1;component/Images/3.jpg" IsHitTestVisible="False"></Image>
</DesignerControlLib:DesignerItem>
</DesignerControlLib:DesignerCanvas>
Can you add a OnMouseLeftButtonDown Event:
<DesignerControlLib:DesignerCanvas Width="600" Height="600" removed="#f5f5f5" Margin="10" OnMouseLeftButtonDown="OnMouseLeftButtonDown" >
<DesignerControlLib:DesignerItem Width="100">
<Image Source="/Kartvizitci1;component/Images/3.jpg" IsHitTestVisible="False"></Image>
</DesignerControlLib:DesignerItem>
</DesignerControlLib:DesignerCanvas>
Hope its help, a little.