Thanks

It's a way to select multiple albums for further actions, like batch-editing metadata or deleting them. It's just like selecting multiple songs. The highlighted corner just tells you that you are in multi-selection mode. Ctrl-Clicking a cover also has the same effect.

In Artist and Album view the images, if you hover over, will fold the top right corner ... what does this mean.
If i click the folded area a check mark shows up and all the other images fold their corners .. what does that mean?