Is your instance set to fixed or responsive width? If it is fixed then you can just adjust the image width accordingly. If it is responsive you can use CSS to adjust your image. I have created banners using simple style properties such as width: 100%; and height: auto; . Also, you can use max-width or min-width to set limits. This will cause your image to resize based on the user's viewport.
A basic example would be <img src="" style="width: 100%; height: auto;">
You would want to use an image at its largest size to avoid degradation.