as far as i know you can make and share any rendered image out of terragen as image/showcase. But as soon as you want to add it into a project you need a license.
So you are fine for sharing even without the license but the ones who would like to use it need one as soon as the project will leave their "house". To be sure check out the eula on their page.

Just point that out the next time and noone should have problems with it....just to be on the safe side in any case

cheers