You shouldn't need to change any of these (except path to PNGOUT if you've installed it).
Instead of deleting old file writing a new one with the same name, just replaces old file's content in-place. This ensures that all hardlinks, aliases, file ownership, permissions, extended file attributes will stay the same.
It's slightly faster not to preserve permissions.
Will write copy of a file with ~ appended to its name. This makes optimisation a bit slower and litters your disk with backup files.
If you have multi-CPU or multi-core system running more than one task at the same time will speed things up a lot.
This will prevent whole system from feeling sluggish during optimisation.
If checked, ImageOptim will try to optimize JPEG files.
Path to jpegoptim executable file. Usually /usr/bin/jpegoptim or /sw/bin/jpegoptim.
If set to 100%, optimisation will be lossless.
Anything below 100% will cause lower quality of files if they appear to have been saved with higher quality setting.
Removes image metadata, like name of program or model of camera which created the file.
If checked, ImageOptim will try to optimize PNG files using PNGOUT.
PNGOUT is not bundled with ImageOptim. It's a great tool, but and author of PNGOUT forbids redistribution of it, so you have to download PNGOUT yourself.
Path to pngout (or pngout-darwin) executable file.
PNGOUT is very effective, but very slow. Use lower level if you're impatient.
PNGOUT may need a lot of time to optimize very large images. This option gracefully interrupts PNGOUT after 1 minute.
Keep it on if you optimize for web. Disable if you have special PNGs with extra metadata.
If checked, ImageOptim will try to optimize PNG files using PNGCrush.
Path to pngcrush executable file.
PNGCrush will not reject PNG files that have invalid checksums
PNG stores information in chunks and many of them are optional. Enter 4-letter names of chunks you wish to remove or press γ button to pre-fill list with gamma-related chunks.
If checked, ImageOptim will try to optimize PNG files using AdvPNG tool.
Path to advpng executable file. Usually /usr/bin/advpng or /sw/bin/advpng.
If checked, ImageOptim will try to optimize PNG files using OptiPNG tool.
Path to optipng executable file. Usually /usr/bin/optipng or /sw/bin/optipng.
Number of different setting combinations tested. It rarely makes sense to try them all.
Interlaced images look better when they're loaded partially, but usually interlace makes compression less efficient.
This checkbox has three states: if checked, will force images to be interlaced. When mixed, it will keep file's interlace unchanged, unchecked will remove interlace from files.
Jpegrescan is a script based on jpegtran. It uses creates JPEG files with unusual number of progressive scans to achieve better compression, but such files may not be compatible with all browsers.
If checked, ImageOptim will try to optimize GIF files using Gifsicle.
Path to gifsicle executable file.
GIF files will be changed to interlaced or non-interlaced automatically, whichever is smaller. In-file comments will be removed.
Please note that Gifsicle works best with animated GIFs and non-animated GIFs rarely can be optimized
For best compression, you should convert GIF files to PNGs (ImageOptim does not do that automatically).