I have a plan to post a lot of images that are free to copy and share for non commercial use on my website. But I was unable to find the actual meaning of non commercial. If my website is displaying ads, does that mean that my website is for commercial purpose? Which means that I won't be able to use those free pictures?
If you site displays ads, yes. Non-commercial use means no monetizing at all. Even for paid URL shortener like linkbucks. It's usually for people who make website just for doodle.