Thanks a lot, Kongondo.
That seems to be right. I'll think twice how to manage it in a deferent way.
For example, if I upload "Martin_Johnson_House,_SIO,_La_Jolla_Shores_-_at_sunset_pano.jpg" I get "Martin_Johnson_House,_SIO,_La_Jolla_Shores_-_at_sunset_pano.640x480.jpg" and I would like to get "rndmname.640x480.jpg"