The value of 255 results in the biggest compression level for the image and the lowest quality. Compression parameter defines the compression ratio for Luminance component of the image and initial value for Chrominance component compression. The real Chrominance component compression value is defined by combination of the Compression and Color parameters. The value 0 means that the Chrominance component is compressed with the same level as defined by the Compression parameter. Negative values improve color quality resulting bigger file size.
Positive values compress color component more resulting smaller file size.
Improved shape positioning. Simplified image preview in Web browsers. Better integration with Adobe Image Styler - Run Adobe Image Styler. It is better to check Transparency and uncheck Make Page options. If in Export settings selected Entire Composition or Trimmed Composition mode then single image will be optimized with Web Image Guru.