Methods, systems, and apparatus, including computer programs encoded on a computer storage medium, for receiving a baseline resource and one or more candidate change resources at a computing device, the baseline resource and the one or more candidate resources being displayable on a display, rendering the baseline resource as an image to provide an image file including corresponding image data, rendering the one or more candidate change resources as respective images to provide one or more image files each including corresponding image data, processing the image files using the computing device to determine one or more statistics of interest, the one or more statistics of interest reflecting a user behavior in interacting with a resource.