simply log in once on another laptop with webcam until you take a photo of you. then log off, clear cookies and log back in your normal computer
I wouldn't log in on any computer that I didn't strongly trust. Keyloggers are everywhere these days. I would recommend buying a high-definition webcam off Amazon for like $50 instead.
i think for one photo is enough a cheap webcam for 10$. Also you can log in from your smartphone/tablet and take a photo with front camera