How to change an image if it's embedded in the HTML?

I am new with wordpress.

I am manipulating the CSS sheet to get the look I want. I noticed the main picture on the page is embedded in the HTML. My understanding is that WordPress does not allow manipulating HTML. So how am I supposed to replace the image? I know it’s possible, since every other site has done it.

My site:

http://www.bmoremarry.com/