There are no defined image sizes in responsive design. The images have to grow and scale on different screen sizes. Here are the sizes that we have found to work the best for the most cases. Keep in mind that the majority of your traffic will be coming from Mobile devices not desktop so it is more important to pick good mobile images.
Headers: 2500x1000
Events: 2000x1138
Blogs: 2000x1138
Podcasts: 2000x1138
Profile Photo: 800x800