class.upload.php is a powerful and mature PHP class to manage uploaded files, and manipulate images in many ways. The script is available under a GPL license.
more info about the class
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Try to use the 0.25 RC1, that you can find here. image_background_color has been greatly improved.
Another question. You know if i can do this:
1. Convert the image to PNG.
2. Make the fill color trasparent.
2. no, this is not implemented, but it is a good idea