To modify "a picture" you can get a trial Photoshop.
To modify "your pictures" you may buy Photoshop OR get a free software.
As Rubble said, it is true that Gimp has lots of options, like Ps.
I do not agree it is "90%" but, it is very advanced and if you get to it's limits, in time, you may afford buying Ps.
A nice and free tool is Pixlr
A basic tool
Also, from Rubble