By creating multiple libraries for your enterprise, you can organize all publications very efficiently so that users can easily navigate through them.
To create a new library you click on 'Content' > 'Libraries' in the top menu,
or on 'Libraries' in the quick overview menu.
Once in the libraries menu you can click on '+ Create new library':
So that a pop-up opens where you can fill in the name:
And in the next tab 'Assets', you can upload an icon that will be displayed in the iPad/iPhone app and the background image for the Windows app:
Tips & tricks:
You can use software like Photoshop or Indesign to resize your image to the exact dimension or use a free online tool like this.