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Setting up power-off icon

I want to set up a large power-off icon on the desktop for a user with very 
poor sight.  I have found a suitable icon and am now struggling with setting 
up the icon on the desktop.

Debian Jessie 8.8 with TDE 14.0.5.

This is what I have so far:

[Desktop Entry]
Comment=Log out and power off
Exec=sudo /sbin/poweroff

saved in /home/$USER/Desktop/

I get an icon (the wrong one!) on the desktop on which I can click and it 
launches the application launcher and asks for an application or URL.  If I 
delete the word sudo I get the menu opened up and get asked which application 
I want.

I asked on the TDE list and was told to set up an con to execute the sudo 
command, then googled, and got the idea for this from:
but am now completely at sea and am probably asking the wrong questions. 

I have never been a lover of icons and am not very hot on their creation and 

The user got badly scammed, so I am doing this instead of just reinstalling 
Windows 7 and letting him get on with it.  If I can succeed, this will be 
much easier for him and MUCH less insecure.