Modifications:

- Added the buttons to the stage. ("upButton" and "downButton" in imageBrowserDocClass, lines 80 - 89).
- Differentiatied between the two buttons using an if statement (buttonClass, lines 32 and 76).
- Added a way for the buttons to stop once the user has reached the end of the images using an if statement (same as above).
Note: The buttons just stop refreshing the images, they do not just start at the other end of the image list.
- Thumbnails and main image refreshes when either button is clicked (buttonClass).

What I'm especially proud of:

It works.