ImageJ is a public domain image processing and analysis program written in Java. It is freely available and used by many scientists around the world. There is a very active ImageJ community and ImageJ is continuously improved. Fiji (Fiji stands for Fiji is Just ImageJ) is a distribution of ImageJ focused on 3D image reconstruction and analysis. For the workshop I would advise to install Fiji, as some things are easier to do in Fiji, but you can use plain ImageJ as well.
