Gimp is open source image editing tool, which can be a replacement of Photoshop.
I personally used this program 5~6 years ago as a replacement of photoshop or paintshop. At that time, this program looked promising but still buggy a lot.
But, current version of Gimp is really improved for the last couple of years. It can handle almost all types of image including gif, jpg, png, even psd, which is Photoshop format.
So, with Gimp, we can work with Photoshop interchangeably. Wow.
This is definitely a good choice, if your company won't buy you a Photoshop, but you still need one.