How to make isometric tile using GIMP (2.8 at the moment)
1. Create a document with a size of 256x256. The size might vary to 32x32 or 64x64 but in general you would like to start with the 32x32 width and height and multiply it by 2 till you match a needed resolution.
2. Fill the image with a color. I am filling it with acid green, you could use any color that you wish.
3. Rotate the whole image by 45 degrees.
4. When you rotate the image don't be surprised that the size of the image has increased. After previous operation on a 256x256 image it is going to be 363x364.
5. Autocrop image
6. Scale image to 256x128. Click on that little chain to break proportional scaling.
... OR Follow the pictures! :D
This type of tile is going to be compatible with Tiled Mapeditor which is a popular tool in 2d game making. Visit http://www.mapeditor.org for more information.
To make screenshots I am using awesome tool called Screenshot Captor. Visit http://www.donationcoder.com/Software/Mouser/screenshotcaptor/index.html for amazing free download link.