There are many image editors for you to choose from. Photoshop is a popular program, but is very expensive and contains many features not necessary for doing pixel art.
Here are other options for you to consider, either free or relatively inexpensive:
- Could use some more options here, Alan.
- Seashore, based on GIMP but with a simplified UI/toolset
- Paintbrush, a simple free editor well tuned for smaller scale images such as starbound assest
- Pixen ($9.99, but designed specifically for pixel/8-bit art)
- Piskel a nice software to make your own pixel based item sprites