It's been opened in Ignite and a new output file, dune.jpg, has been set up. Of course we could just click the Save this output button on the Inspector and have Ignite automatically save the JPEG with the settings it has chosen. But if we want to take control of the optimization process then we need to look at the JPEG file pane of the Inspector . This is the standard quality slider found in every application which compresses JPEGs.
Lower quality reduces the quality of the image, and so produces smaller files.
By default, the colour is encoded at the same quality as brightness, but using this slider you can change the colour quality independently. Since JPEG compression works best on images which are composed of smooth changes in colour, this can be very effective in reducing file size. However, it will affect the clarity of images, but a little smoothing may reduce file size and be hardly noticeable, especially at low quality settings. Standard JPEG compression applies the same quality setting to each 8x8 block in the image. However, in most images, some blocks will have more detail than others, so it's a bit of a waste if the low detail blocks have the same quality as the high detail blocks.