Google+ tech lead Dave Cohen announced on Google+ this evening that the service has rolled out a new feature for interacting with high res photos. It's a pretty elegant implementation for navigation: a mouse wheel or two finger swipe will zoom you in and out, while clicking and dragging lets you pan around the image. A map of your image appears in the top left of the screen, showing you exactly where you are as you begin zooming in and panning around. Hovering over the map box with your mouse also gives you the option to zoom in and out and select specific sections on an image.
A mouse isn't necessary though — testing on a MacBook Air, swiping left and right or up and down with two fingers on a trackpad zooms in and out, while clicking and dragging moves you around the image. Overall, it works quite well, and there's no discernable slowdown moving around the 2MB image.