The bundle header image should be at least 1100px wide and no larger than 2MB.
The file size limit is 2MB for the bundle product image and 2MB for the bundle header image.
If the image you upload is larger than this, the bundle will not save.
Please note that the larger the file size the longer the bundle page may take to load. Please ensure your images are optimised for web use.
You can find your bundle product image and header images here:
Apps → Bundle Builder
Choose your bundle
Under “Setup” you will see "product image" and "header image" options