This has been asked a lot...
to save repeating all the answers here,
I suggest that, becuase most of the answers to this question, include a package called "gimp", the best idea is to search the forums for the word 'gimp' and you will find most of the questions and answer about which graphic package people use and why different people like different ones..
If you still don't find the answers you need, or need a more specific answer after searching, simply ask .. .we are here to help