If you use minimenu it is easy, just make an icon with dimensions of 60 x 60 pixels as a .PNG with the name of the .png being the same as the app(appname.png) file and then put it into the same dir as the PND.
Same for the preview pic if you make a pic 285 x 180 pixels and name that the appname_pvw#0.png. I also found that if you use these preview pics they load MUCH faster than the ones imbedded in the .PND