Save your customised cover image in your Photo Album then upload it in Noteshelf in two ways:
1. For new notebooks, from the + icon on the Main Shelf, tap on "Pen icon" on the Cover/Paper then swipe all the way to the bottom of this selection screen to find the "Custom" option. Here, you just need to tap on 'Plus' icon to to upload your desired cover image from your Photo Album.
In order to edit this custom cover/paper's title color and position later, you just need to do a long tap on the custom cover/paper and tap on the pen icon.
2. For existing notebooks, tap and hold on one or tap on the Edit icon from the Main Shelf then tap on the Edit icon on desired notebook. From there, it is the same procedure as above.