Ok. I got it. The semicolon wasn't the main problem: I haven't said in "Exec" that my program accepts urls or filenames. I've solved changing the line of Exec by this one:

Exec=/usr/bin/pimagizer %F

Thanks to all.

2013/4/12 Matthias Clasen <mclasen@redhat.com>
On Thu, 2013-04-11 at 23:39 +0200, Víctor Fernández wrote:
> This is the file:
> - - -
> [Desktop Entry]
> Encoding=UTF-8
> Name=Pimagizer
> Icon=/usr/share/pimagizer/pimagizer.svg
> Comment=Resize images using Python
> Comment[es]=Redimensione imágenes usando python
> Exec=/usr/bin/pimagizer
> Terminal=false
> Type=Application
> Categories=GNOME;GTK;Utility;
> StartupNotify=False
> MimeType=image/jpeg;image/png

You are missing a semicolon at the end. Thats the way lists in desktop
files work. Compare Categories.