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: