As an artist, you probably will be approached by various people over time. Some may be looking for commissions, others will most likely be looking for free art.
Here are some handy resources concerning the "free art"-people. First of all, there's this handy flowchart called "Should I Work For Free?.
If they say they'll pay you with:
-Eventual cut of the profit when profits are made
-No pay, since artists "shouldn't be sell-outs, but draw for fun"
Then run for the hills and get away as fast as you can.
Because if it's for a game, a comic or another for-profit project, the most normal response should be "Fuck you. Pay me." (worded politer, though that goes without saying).
Spec-work? -- Fuck you. Pay me.
"We don't have the budget for 20 sprite-sheets, but we'll sell this game and your name will be in the credits!" -- Fuck you. Pay me.
"We need 10 sprites / full paintings by tomorrow,