A small suggestion: it would be useful for your tool to display not only the thumbnail links, but links to the image pages on the NASA site as well.
Not sure to understand.
When you select a pair, it displays the assembled image and writes links to the full size images at the bottom.
For an unknown reason (probably something related to how the hosting service stops and resumes the apps), it was rolled back to a previous version.
This version had a bug that may affect those links. Seems to be okay again.