Modding:Image Editing
From Starbounder - Starbound Wiki
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:
Cross-Platform
- Aseprite is what Chucklefish uses. It's payware, but you can build it from the sources for free. They have packages in Ubuntu's repos, and Arch's AUR.
- GIMP (free, open source)
- Pencil
- Synfig Studio
- Pinta
- Piskel
Windows
- Paint.NET (free)
- Graphics Gale (Shareware, $19.15 for full version)
- Pyxel Edit (in beta, currently $9)
Mac
- 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
- Pyxel Edit (in beta, currently $9)
Linux
- GIMP [free, open source] (Available thru apt)
- Krita[free, open source] A powerful and free digital drawing/painting program, but with feature support for pixel art.
- Kolourpaint [free, opensource] Simple and basic image editing software similar to MSPaint (Available thru apt)