The image you decide to use of your artist must complement the colour scheme of the album cover and the style of the artist. E.g. Rihanna has taken a dark theme with the heavy make up which can be seen through the black and white filter. The image complements the black and white use of text.
The visual style of the inside of your digipak should look very similar and should complement the front of your digipak, with regards to the colour, the design, the font.
Acknowledgements should also be included in very small font- acknowledgements for particular tracks on the album.
A repeat of the track list is sometimes included. But with more info about the length, songwriters and producers.