This is not a Liferay-specific problem - it's rather a web application problem. There are plenty of hits if you search for "HTML5 camera access" or related terms. Even Stackoverflow has a lot of them, e.g. here and here. The questions are older - in this case that's good: HTML5 only got more wide spread and while not everything might have been supported back when the question was asked, we're a lot further along the way.
I have no experience with this integration - I'm suspecting you'll have to make use of <portlet:resourceURL/>
in order to process some of the input through Ajax when you want to process the results in Liferay, on the server side. But the basic problem, accessing the image from a web page, has a pure HTML5 solution, totally independent of Liferay.