Hi Sara, if you’re comfortable with CSS, I’d encourage you to experiment to get the effect you’re after.
The class you’d want to target is .entry-content. You’d first need to make your file smaller to better fit the width of the content area. Then, this is some very basic CSS code you could start experimenting with:
.entry-content {
background-image: url("https://178.239.177.177/wp-content/uploads/2013/09/your-image.png");
background-repeat: none;
}
More about the CSS background property:
https://www.w3schools.com/cssref/css3_pr_background.asp
You may also need to adjust some of the theme files directly if you need to add additional classes to get the image appear to stretch vertically to accommodate more content. I don’t know how advanced you are in theme development, so if it’s not something you’re comfortable doing yourself you may want to hire someone to help get it exactly how you want it.