For your computer-generated pictures, try picking another colour instead of black for your backgrounds. You've already got PLENTY of black in your picture, and if someone hadn't looked up close at your picture, it wouldve looked like a big black canvas with blobs of white.
The thing you've got with the white around the characters to seperate it from the background is working, but it's a bit un-even looking and unpredictable. It looks grainy ... next time, when you draw your characters, draw it on a new transparent layer above the background (if you want it black) and when you're finish with the characters, give it a white outline (Photoshop has a function for that. I'm not sure about any other softwares, if you're not using photoshop.)