This video sets out the concept for posting blog or news articles to your Joomla site. It explains why they should be structured in a particular manner with Read More Links instead of having complete articles on the News or Blog page in your site.
Structuring your blog posts in this manner will do a lot for telling you which posts are attracting the most traffic.
Free images for use in blog posts can be found at http://www.pexels.com and https://unsplash.com/ but there are several other free image sites around.
May your blog posts bring you much traffic and lots of quality comments.