Thanks to all !!!
The image has been given to me in .jpeg format, but I can ask the writers to give me a .png or a .gif. This is probably the best solution.
I tried the .svg container but it does not solve the problem.
Actually, what I would really like is to work directly with .svg images (this kind of plots are easy to be drawn with Inkscape or similar) but I am not really sure that all readers (including my Sony PRS-650) are supporting them.
So far, and having only the .jpeg, I have resized the images to obtain a good result on ADE full-screen, and on the readers I hope a built-in zoom function will be of some help (the one of the PRS-650 does a good job).
No way of obtaining a good result with scalable units.
I'll let you know how it works with .gif