Micheline can't erase photos on the iPad from Picasa. Leo says that there's a setting in Picasa that will allow it. Leo also says that Windows photo sync is terrible and iTunes on Windows isn't that great either. That's why Picasa is a better option.
Ideally, though, iTunes and the iPad work best on the Mac. Here's a link that talks about issues and various solutions. There's a photo cache called iPod Photo Cache that she can delete.