******PROBLEM FIXED****** I found yesterday a way to avoid the problem. First I launch PyMol, then I change the viewport to the one I have in my script; and finally I run the pml script. This gives full, uncutted png images.
Btw, does anybody know if there is a python command to wait until a process/command execution is done? I could include this kind of command in my script just after the viewport command (In VB .NET, this command is "application.doevents()"). Greg.