Image editor

Most games need some graphics. Unless you plan to draw absolutely every element in your game as flat shaded 3d primitives, you'll need textures, custom mouse cursors, and who knows what else.

The best paint package I've ever used was Cloanto's Personal Paint. But that was on the Amiga more than a decade ago, so unless you have one of those lying around (or UAE) there are a few more modern options.

1. Photoshop. The grand daddy of image editors, but expensive and burdened with what I find to be too many features. But then again maybe I just sucjk at photoshop...
2. The Gimp. Apart from the scary name that got me a few funny looks from my wife when she saw it on my desktop, a pretty good editor for programmers.
3. Online editors. These are getting better and better. No installation, so if you find yourself at the in-laws over thanksgiving or Christmas you can still knock out a few icons or textures. I like pixlr and splashup. 

I would recommend spending only as much time drawing your graphics as you find fun. It's really easy to go back and improve them - and it's also really easy to change the design or display method of your game so that it needs different graphics, so don't commit too much time up front. For this reason I recommend starting with pixlr or splashup just to creat some stand in graphics, then revisiting then later with the Gimp. If a guy in a suit with a briefcase full of cash turns up and wants to publish your opus, then it's time to splash out on photoshop.

In any case, your goal will be to produce, in most cases, a 24-bit .png file with an 8-bit alpha channel. This is a really flexible format, well supported by both image editors and java. That's why PNG stands for Portable Network Graphics!