How do I add an image to a post?

Pardon the (probably) dumb question, but I’m brand new to WordPress. Just installed it today, and have installed the Ignite theme.

Theme looks great, but I’m wondering how they added the image shown on the preview. I don’t know if this is an image for that post (so you could have a different one for each post), or if it’s the same image for every post. Either way, can anyone tell me how it’s done.

Thanks a lot!