What color in a png format image would be transparent?

I'm in the beginning stages of my game. I was wondering on how do I create something transparent in an image? Is there a specific color that makes LOVE ignore that color and make it transparent?

Greetings, My name is Edward. I am a student in UCI. I am currently taking a project course called Game Development. We, my team, want to make a game like Maple-Story, with cartoony graphics of small eggs people (with swords and shields and stuff). We are thinking about using the Love engine, it's a...