The comic was printed by Image comics and they have not released it in any electronic form.

I maintain that if you have not played the first game you can still follow the story by playing the second one, there is a recap at the beginning of the game that covers all the important plot points. Yes, you do miss out on some of the funny banter or the more substantial introduction of characters but you get what you need to know to make sense of the plot this game.

The secret origin stories for the heroes are included in the game in the training/upgrade section for each character (Liberty Lad excluded, as his was a level in the original game but there is a write up of it on his info tab in the training section)