The "Best Fit" button scales the Active Image picture to fit the window.
The "Full Size" button displays the Active Image picture at it's actual size which may be larger than the window. In the "Active Image" window, use the mouse wheel to zoom, or click and drag to reposition a picture larger than the window.
The "Back" button is similar to a "Back" button is a web browser. After you click a link in the "Text" tab of the upper left "Active Image Information" window, the "Back" button returns to the previous picture.