It is said that MSPaint takes command line switches /p, /pt and /wai (although no one has an idea what the latter two are for.) To my knowledge, when MSPaint is launched with /p switch, it prints the specified file out of the printer and then exits without showing the window. You can right-click a bitmap file on the explorer and choose "Print", with this function.
Anyway, MSPaint translates the command line on its own, and GM has no control over it. So, you should consult other places for it, e.g. MSDN, rather than GMC.
Im using execute_program to open MSPAINT with the file I want to edit but want it to automatcally zoom in X8 when opening the file.
As far as I see, there is no description about the command line switch in the help file of MSPaint. So, it basically isn't meant to provide flexible control for users with the command line, except a few ways to launch it from the explorer. So, I highly doubt that MSPaint provides such a function via command line switches.