Gallery can process images with either iMagine or Graphic Converter. There are a number of options available to you, and you can choose to apply the tweak to the thumbnail, the main image or both. Note that these tweaks will affect the processing time for each image, so the overall gallery process time will increase.
iMagine is an excellent application from Kevin Meaney - one of the Softpress developers. This application provides AppleScript access to the graphic processing capabilities of Mac OS X, and offers many more options to the user. As such, the Gallery will offer different options to iMagine users to those using Graphic Converter. I recommend that all Mac OS X users switch to iMagine.
NB - iMagine is a Mac OS X only application.
You can get GraphicConverter here. Don’t forget to register this excellent application!
This converts the image to a grey scale. iMagine suers can also tint the image in a number of ways.
Alter the brightness/contrast of the image.
Some images suffer badly from reduction. This can help rescue images.
This inverts the colours of the image.
More information on these features can be found in the GraphicConverter manual.
Note that this can impact on processing time - so if you are doing the full 250 images, you could be waiting for a while!