Thanks for the mention of HeroMachine! I just wanted to point out that HeroMachine 3 (http://www.heromachine.com/heromachine-3-lab/) allows you to export as a PNG directly, even at very large sizes. Or you can export it as a transparent PNG at I think 400x600.
Just don't fill in any of the colors and you'll have a nice crisp black and white image to use for practice.
Good luck, and have fun! As a side note, I drew all of the artwork in HeroMachine with a Wacom tablet directly into Flash. I use the same basic technique for doing all of my color work, either exported to Photoshop or natively in Flash.