Cube Tutor was taken down at approximately 22:00 GMT tonight on an emergency basis because our card images were no longer showing up. We were using the service provided by MTGImage.com which is no longer available. You can read more about that over on their website.
This means that for the time being Cube Tutor has been migrated over to Gatherer for card images, however we are somewhat at the mercy of Gatherer as to which edition of each card shows up. This is (hopefully) a short term solution until we can find something to better fill the gap left by MTGImage, which includes all of the promo images and other card versions.
We have not had time to thoroughly test this change, but it seems to be working for the most part. If you notice any missing card images then please get in touch.
In the mean time, please bear with us while we look for a better solution.