Priyoban - Yes, exactly that & leeching bandwidth has two major issues :
1. Everytime we open your thread, the images have to be loaded up from TT, so say TT servers are slow for that day - the page loading time of your thread would also increase dramatically. Not to mention, stealing b/w is considered by many as not the wise thing to do.
2. What if TT swaps those images with say a "p0rn image" - & this scenario has happened on TE before. That's why we ask you to always upload your images to a image hosting server or use our own attachment feature before posting on TE.
Irrespective of the site in question, what's wrong is wrong - whether its TT or NASA is immaterial. Hope you understand & would take care next time.
As you said, if you had mentioned the source & not hot-linked images all would be fine.