Take your application's 512x512 icon and make a copy in your ad-hoc's build folder called iTunesArtwork (no extension).

#!/bin/bash
rm -r Payload
mkdir Payload
cp -rp myapp.app Payload
rm myapp.zip
zip -r myapp.zip Payload iTunesArtwork
rm -r Payload
rm myapp.ipa
mv myapp.zip myapp.ipa