Gimp has all of the tools that you are used to from other photo editing apps like Photoshop. It does an amazing job of keeping up with the times and adding in new tools. The animation add-on for the program is also pretty sweet. I use this primarily on the Mac and up until a couple years ago it was a bit of a bear. They added in single window mode though so everything is now a lot more manageable.

I feel it is easy to install and it has lots of features which make it one of the best software. And most importantly it is free. It is faster than other photo editing softwares and consume less space on HDD.

GIMP has most of the features you'd be wanting from an expensive program like photoshop, but without the cost.