[Q] Which is the best Format for the Bitmaps Ressources ?

What is the best Graphic Format (PNG,JPG...) to insert images?

Both PNG and JPG will work. The images are compiled into a format that the device can use so the initial format doesn't matter as much (there's no dynamic scaling of the image).

Which resolution should we use for the images? (low bit-depth images ?)

Flatter images will look better. A lower bit depth is a good idea as the image will be converted down to a 4 bit image on the fenix 3.

Is there a color Limit regarding the generating of Bitmaps for the fenix3? (I think 64 colors?)

There's no limit to the number of colors you can use. The fenix 3 has 16 colors (15 + transparency) which you can find in the devices.xml file in the bin folder of the SDK or the User Experience Guide (I think the purple on the fenix 3 is slightly different than the other devices). You can use colors other than those defined in you image but the compiler will simply map them to the closest supported color.

For what is the unique plaette needed?

You can restrict the compiler to use a specific set of colors when compiling your image. This will save memory (3 colors + transparency only requires 2 bits per pixel). It may also make your images dither better if you want to prevent certain colors from being used while dithering. You can also disable dithering so each pixel gets mapped to it's closest matching color.