When you click on an image and choose edit, there's a Title field there that you can fill. That will add it to the
alt attribute, which is a bit confusing perhaps for technical users looking for the alt tag.
If you want an actual
title attribute, I don't think that's currently available from within Redactor. IIRC, Redactor 3 does support figure/figcaption as a future alternative, but, before you ask, I don't have a release date for that just yet