The Design Assets section is where you can add the pictures that will appear as the Shop Header and ‘Splash Screen for your app.
Firstly it is recommended that you upload the pictures you want to use using the ‘Pictures’ section of CMS, you read more about uploading pictures here.
The recommended size for Splash images is: 629 x 1365
For Shop Header it is: 749 x 392 (or similar ratio)
The ‘Design Assets’ page lets you manage the Splash Screen and Shop Header of your app. You can manage your ‘Design Assets’ in the ‘Content’ section of your CMS.
To upload an image for the Shop Header or Splash Screen simply select ‘Select Picture’ beside the appropriate section and choose the image you wish to use.