1. A method comprising: displaying a first digital image of a first tissue slice on a graphical user interface such that an area of the first digital image is enclosed by a frame, wherein the displayed portion of the first digital image is larger than the frame; displaying a portion of a second digital image of a second tissue slice on the graphical user interface, wherein the first tissue slice and the second tissue slice are both z slices taken from a tissue sample at corresponding positions in the x and y dimensions, wherein the displayed portion of the second digital image has a shape of the frame, wherein the area of the first digital image that is enclosed by the frame corresponds to the displayed portion of the second digital image, wherein the area of the first digital image that is enclosed by the frame is co-registered with the displayed portion of the second digital image; and shifting the displayed portion of the second digital image in the x and y dimensions to coincide with the area of the first digital image that is enclosed by the frame as the frame is shifted in the x and y dimensions over the stationary first digital image.