On some of your pages, you might have a Banner images which shows up across the entire top of your page.
Updating these is very simple and is explained here.

Once you on the back end of the page you want to edit, you can either mouse over or click on the top banner image.

Next click on the settings icon located in the top left corner of the page here

Once you’ve clicked on that, a pop up will appear.

Click on the drop down arrow to the right of “Background” here:

Next if you are looking at changing the photo, click on the image you see showing up there:

This will bring you to the Media Library where you can select an existing image you have previously used on your website.
Or you can click on “Upload Files” to select an image from your computer.
Banner images are generally larger photos so if you are trying to upload a very small image then you may have issues with how it looks. So make sure that you use a photo which is large enough to look good as a Banner image.

Make sure to save the changes once you are finished up by clicking the green check mark.