Images make up 62% of average total page size: 812kb of a total 1311kb as recorded by Steve Souders' HTTP Archive as of March 2013.
As the major part of total page size, images have a high correlation of 0.71 to total page load time. This makes them an ideal target for performance optimization.
But despite the high ROI of image optimization, average total image size has grown considerably: in 2012 the total image size grew by +28%.
While reasons for this growth vary, ranging from large header designs to retina displays, its consequence is simple: image optimization is becoming ever more important.