On the first image I see a very good use of Anatomy and perspective wich would be much more atractive with a bg since each character has an especific location and they're not just placed one next to other.
Again, try to be carefull with the lineart, before moving on adding color, carefully look at every corner of the picture and erase any outline, example: The girl at the back, right hand, an outline is visible just beneath the microphone.
Overall it's a really good picture with good depth and a solid idea that would look exellent with a bacground
On your second picture... silly nozomi, you posted the same link
