Most Cisco IP phones have a built-in screen capture utility.

On the SPA525G2, for example, just go to:

 http://x.x.x.x/admin/screendump.bmp 

Where x.x.x.x is the LAN IP address of your Cisco IP Phone.

Wait a second for the phone to render the snapshot, and Ta-Da!

Different models might have slightly different URLs.

Example Screen Capture: