GIMP (The GNU Image Manipulation Program)

GIMP stands for the GNU Image Manipulation Program. It's basically a graphics program that allows you to edit images. 

It's free, and it's way more advanced than MS Paint (duh). It allows you to make transparent backgrounds, do gradients, draw animations, etc. In fact, there are most likely a lot more advanced features, but the ones that I mentioned are enough for a highs school amateur like me.

To be honest, the interface is confusing—it's almost impossible to do anything without reading a guide. In fact, I couldn't draw a circle or a straight line without the help of the Internet! Fortunately, this program is pretty popular and usually just by typing "GIMP Task_Name" you would be able to find an appropriate guide.

Once you get used to the interface, this program should be quite easy to use and useful. I have used this program for several projects and I have gotten quite good results. If you haven't already tried it, try it now!   

Official website: http://www.gimp.org