/sdcard/DroidScript/MyApp/Img/MyApp.png
The easiest way to do this is from the assets tab in the Wifi IDE - http://wiki.droidscript.me.uk/doku.php?id=getting_started:wifi_ide#using_uploaded_images
Once the apk is built, there is no straightforward way to change the main icon
You can tell if is in the right place by shutting down DroidScript and restarting it.
The project produce should then show that icon in the main DroidScript screen