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:
Hero Image
The image should be in 4:1 image
Image will be converted to a JPEG in the end and the final dimensions will be reduced
ex. 6000px by 1500px will be reduced to 2400px by 600px
Logo
Image size should be at least 300px by 300 px but uploaded image sizes as high as 4000px by 4000px will work.
Main Image
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.