With your Sun West Adobe license, you can create images easily with Adobe Spark.
To create a custom sized banner for your school website, go to https://spark.adobe.com/
Click on the plus icon to start a new project. Here you will see suggested templates you can use. To create a custom sized image, scroll all the way to the bottom and click on the Create a graphic button.
This will allow you to create a custom sized image to fit the website banner dimensions. Click on "CUSTOM" and enter the banner dimensions of 900px x 413px.
Now you can add your photo, text and save the image!
Once saved, you can upload to your Sun West School Website.