Quick And Easy Graphics

Hey everybody im new to the site and i have an important question. I need a basic graphics program thats freeware. the main purpose of this program will be to upload a picture and play around with the colors and pattern of the design i want. PLEASE HELP if you know of such a thing. I know this is macs territory being its creative stuff but i cant find one thats just simple and reliable.

thanks

You might want to check out The Gimp