Set the image quality per image style!
Through Image Toolkit, you can set the quality of images the Image module produces. But since Image Toolkit allows you to set only one global value, any image produced by Image module ends up in the same quality. If you want to have the highest-quality images in a photo gallery while having lower-quality images elsewhere, you are stuck.
This module allows you to override Image Toolkit's quality setting and set image quality per image style. Unfortunately, since ImageAPI does not let other modules hook into / alter the process, you need to apply the supplied (unobtrusive) patches to Image module and image.inc in core.
The patches basically only add optional arguments to existing functions so the Image module would function as normal (as far as I am aware) even if this module is unused.
#1310452: Allow to override the image toolkit's global JPEG/PNG quality value
Because this module requires a patch to be applied to the core, it wouldn't become a full project until the patch is accepted and committed (or an alternative solution is introduced to the core which would allow us to update the value cleanly).
Please remember to always flush the cached image every time you change the setting, if you want to see the effect immediately.
Imagecache Quality module is available for Drupal 6.