Guys I read all your answers but I think there is one plugin that you've missed; and I am talking about the Jetpack.
This is an amazing well made plugin made from Automattic and from the others it gives you a CDN just for images and it will resize them as well, I advice you to read their API because it gives you so many ideas ;)
Beside that since the 4.4 version you souldn't worry anymore even of the pixels dimension of your images because from now on WordPress supports the srcset attribute to serve different sizes on different devices.
Hope this gives you a little overview of Jetpack because there are plenty of useful feature inside this single plugin ;)