Converting a short image-resizer script from GD Library to ImageMagick

I got the attached script on my osCommerce website. All it does is taking images and re-sizing them, while applying watermark and caching them to browser and my server. The short script is using the GD library, and since the output of a downsized image is not sharp enough (kind of looks like it is using something similar to bicubic smoother algorithm) I would like to convert it to use ImageMagick instead as it can downsize while using "Bicubic Sharper" method and maintain the sharpness of a reduced-size image. I want to keep the script as close as possible to its original form and certainly to keep all the watermarking & caching and other functionality it currently has. This should be a very simple job for a professional who's experienced with such work, and should take very little time to accomplish, so please bid accordingly.