You can’t just add any picture to your campaign header, logo or background. In order to work best images need to be certain dimensions and file sizes.
Logo: 100px x 100px exactly.
Header: 175px height
Background: 600px width
Before uploading your images, make sure you compress them to the smallest size possible. If file sizes are larger than they should be and depending on your internet providers connection, they may not show as they are suppose to. Also, do not name your photos with long names. Make the name of your image as short as possible.
Go to the link below to compress files easily.