There are a few different images you can upload for your Company Branded Page here is some information to set the process up for success:
The focus of the image should be at least 1845px by 450px dimensions
This will need centered on a 4:1 image
Image will be converted to a JPEG in the end and the final dimensions will be reduced
Image size should be at least 300px by 300 px but uploaded image sizes as high as 4000px by 4000px will work.
You can upload any size and the window will be in the ratio of about 46:27 so if you create an image specifically for it something like 460px by 270px or 4600px by 2700px would be best.