if there's any alternative to phpThumb that doesn't require it at all.
Try taking a look at Imagine, it's an Image manipulation library for PHP 5.3 inspired by Python's PIL and other image libraries.
It has drivers for GD, Imagick, and GMagick. It should provide most of the functionality phpThumb offers and does not require shell access.
Here's a nice presentation and supporting documentation.
Good luck,
Anthony.