Have you hear of Photoshop? Who hasn’t, right?! Although this piece of software is incredibly good, it is too powerful for the vast majority of users. Why spend so much cash on such software if you’re only using about 10% of its real functions?
GIMP (GNU Image Manipulation Program) handles image manipulations task extremely well. It works for the newbies who just want to resize or convert an image file, up to more advanced users who would like to build complex images. The main principles of image editing are present in GIMP (layers, channels, and a lot more that I haven’t even explored). Another nice thing about GIMP is that it works with plug-ins so you can extend its functions.
Here you can cut, paste, convert, retouch, resize, build images, and take screenshots of your desktop. You can even open and edit PDF files with GIMP.
This program is quite simple to use, as long as you are familiar with some concepts of image editing.