Marco,
You'll need to rename or move the file to a different <name>.ppm
file. You can do this individually, or possibly within a script
that is creating your images using p.map using varibles that change.
Just an idea 
I would like use p.map to create a <name>.ppm in a <DIR> but this
command create only paint.ppm in HOME directory. I'd like change the
output name and output directory everytime.
Thanks
Marco
I hope this helps.
Louis Clarke U.S. Army Corps of Engineers
lclarke@alb-co-r.swa.usace.army.mil Albuquerque District
voice (505)342-3281 Regulatory Branch
fax (505)342-3498 http://www.swa-wc.usace.army.mil/reg/