What Java Runtime Environment should I install to get Icy working ?
Current version of Icy requires Java 8 at least to work, see the download page to pick up the appropriate Java version.
If your system is limited to Java 6 then you can try to use Icy 1.8.6.0 which is the last version supporting Java 6.
If your system is limited to Java 7 then you can try to use Icy 2.0.3.0 which is the last version supporting Java 7.