Gwenaelle,
As I (mis)understand it you have a raster image.
You may create a rectangle of other path with the desired gradient, then move the image on top of it and in the Transparency palette (flyout) click Make (Opacity) Mask with both Clip and Invert Mask ticked (in the main Transparency window).
Or you may redraw or Image/Live Trace (and Expand it) to have it changed to vector artwork, then apply hte gradient.