The main article image you see in Skyline News theme is called featured image. They are used in article listings – on front page, archive and search listings.
For example in the article image of this particular post you see a woman’s hand holding a phone. It is a featured image of this article/post.
Adding article images is easy. There is a small dialog box titled “Featured image” on the screen when you add a new post. It’s normally located near the lower right corner of screen. If you don’t see the dialog, make sure you have checked the “featured image” option under the “Screen options” tab. See the image below for details (click to see in full size).