Use this tab to set a compression value and chroma subsampling for the file.
The compression action reduces the file size by deleting image information. This means that as you increase the compression, you lower the quality of the image. Use the image preview window to help you find the best balance between the two.
With chroma subsampling the file size of the JPEG is reduced by averaging the color information for every 2 x 2 square of pixels. You can change this setting to have a larger area of color information averaged.
Previewing images in a Web Browser