De acordo com o site ImageAlpha :
Batch processing
ImageAlpha is mostly based on pngquant. You'll find compiled pngquant executable in ImageAlpha.app/Contents/Resources directory.
por exemplo,
ImageAlpha.app/Contents/Resources/pngquant -ext 8bit.png yourInputImage.png
Para mais opções, execute:
ImageAlpha.app/Contents/Resources/pngquant -h
Goodluck