You can always set photoshop (or other graphical apps) in the preferences. Goto Window -> Settings/Preferences and then select Applications (at the bottom) you can then set up maya to open PSD files of "other images",

I like to use tif's I can then paint the UV's in PS apply it to a material that is applied to the model then paint and reload as next said. tif's allow you to use layers, when your working and update nicely.