Gimp (GNU Image Manipulation Program) is a powerful, free answer to Photoshop. I use it for all my thumbnails and layered images, and have even used it to create assets for Unity. Below are some helpful resources and hotkeys for Gimp.
Gimp Hotkeys:
- Invert Selection – CTRL + I
- Swap Colors – X
- Color Picker – O
- Text – T
- Zoom – Z
- Undo – CTRL + Z
- Cut – CTRL + X
- Paste – CTRL + V
- Paste in Place – CTRL + ALT + V
- Duplicate – CTRL + D
- Save – CTRL + S
- Save As – SHIFT + CTRL + S
Gimp Resources:
- Gimp Home Page – You can download Gimp here.
- Gimp User Manual – Pick your language and then read away.