Capture screen of website and make it avatar

My website allow businesses to promote their website and they have to update their website URL in the control panel. They normally upload their logo or banner to become thumbnail picture (avatar). But some business does not have their logo/banner ready at the time, so as soon as they update their website URL, our website will capture the screen of the website to make it avatar as default then they can upload their logo or banner later I want a piece of php code that can do this screen capture work and integrate into my existing website.