How can I convert a .jpg or .png image to .raw format?

I just find out here how to do that with Gimp !

Works almoust perfectly ! :P

Update

Used method:

1. Export the file on Gimp: Ctrl Shift E

2. On the export window, choose Raw Image Data on Select File Type

   1. Click on Export button
3. On the raw image export window, Choose Planar for the RGB Save Type
   1. Click on Export button
4. Open the folder where the file was saved and rename his extension from .data to .raw
5. On Unity3d, chose Bit 8 for Depth on the Import Heightmap window

Unfortunately, the generated image by Gimp isn't that good... So, I'm still would appreciate some help.

Ubuntu by default has Imagemagick installed and provides the convert command line tool to convert to various image formats. You can simply do:

convert xyx.jpg xyz.RAW