XnView Software

This features has a great "preview function", but I'm not sure if all users will realize this. So maybe you could add a small description - see attachment.

sorry, but it's obvious

It's obvious that there is "some preview".
But it is not obvious that it is not a static, but a dynamic preview and that there is a difference between "selected" and "marked" features.
It's not obvious what happens when more then one effect is selected.