Try below trick and create the icon to lock your PC.
1. Right click an empty spot on the desktop, point to New and than click Shortcut.
2. In the Create Shortcut dialog box, type the following :
3. Click Next.
4. In "Type a name for this shortcut" (any name as u wish) and Click Finish
5. Create a shortcut in your shortcut bar & use it.
6. You can change icon by Right Click > Properties> Shortcut> Change icon