I would recommend adding it as a widget since you probably aren't using a child theme (any updates to the theme would override your edits unless you put those in a child theme).
Upload the image. Get the path of the image after it uploads (click Edit
in Media Library
and the path is on the right side in a greyed well).
In the Appearance > Widgets
area in the admin section, add a text widget above the sidebar widget that is creating the Of Interest
section and then include the code
<img src="path/to/image.jpg">
To hide on small screens you will need to add a media query like this:
@media screen and (max-width: 767px) {
.hide-mobile {
display:none;
}
}
You can change the width px to whatever your preference is. Devices vary.
Then you would append the class to the URL like <img src="path/to/image.jpg" class="hide-mobile">