Is it possible that ... webcam captures the image?
That's not how it usually works. Usually, the USB host machine (presumably a PC) reads the image from the web cam and captures it in some form. In other words, the 'capture' part is performed by the PC application and not by the web cam. This implies that your problem is to find/create a PC application which can read the image from the web cam on demand, and provide a mechanism for the Arduino to tell it to do that. There are plenty of freeware applications which you can run from a command prompt to do this, and if you're using Microsoft Windows then you could use Gobetwino to run the application when the Arduino commands it.