These are the conditions for each set of bonus art...iirc if you get SS in Easy all the way you also get the S rank ones too...
Clear Bonus Art #1 Complete the game on Easy
Clear Bonus Art #2 Complete the game on Normal
Clear Bonus Art #3 Complete the game on Hard
Clear Bonus Art #4 Complete the game on Very Hard
Clear Bonus Art #5 Complete the game on Dante Must Die!
Clear Bonus Art #6 Obtain an S rank on every mission in Easy
Clear Bonus Art #7 Obtain an S rank on every mission in Normal
Clear Bonus Art #8 Obtain an S rank on every mission in Hard
Clear Bonus Art #9 Obtain an S rank on every mission in Very Hard
Clear Bonus Art #10 Obtain an S rank on every mission in Dante Must Die!
Clear Bonus Art #11 Obtain an SS rank on every mission in Easy
Clear Bonus Art #12 Obtain an SS rank on every mission in Normal
Clear Bonus Art #13 Obtain an SS rank on every mission in Hard
Clear Bonus Art #14 Obtain an SS rank on every mission in Very Hard
Clear Bonus Art #15 Obtain an SS rank on every mission in Dante Must Die!
Hope this helps
Good luck btw