Optimization of JPEG images

• The image compression techniques have their origin in the necessity of reducing the size of the file used to store the image, witch is huge when compared to storing text (a 800x600 24 bit color uncompressed image needs 1440000 bytes or 1.37 MB). One of the most popular formats is JPEG (which uses files with JPG or JPEG extensions), which can achieve compression ratios of 10:1 with almost no perceived loss of image quality.
  Depending of the program you use for saving JPEG images, there are available some options for adjusting the compression of the image. In this article I'll try to explain how to adjust that options and optimize the ratio between image quality and file size. It's important take into account that some images are more suitable than others for JPEG compression (see flat images compression below).
• Basic compression
  optionsThe two basic options that are almost always present in every "Save JPEG" dialog are the compression/quality of the image, and the standard/optimized/progressive formats.
  Both compression and quality refers to the same parameter but one is the inverse of the other (e.g. 35% compression is equal to 65% quality), depending of your program choice you'll find either. The more compression, the lesser image quality. The proper setting for this parameter depends of the image quality or file size you need/want to obtain, getting the best ratios with 50%-85% of image quality (15%-50% compression).
  The standard (baseline) and optimized (baseline optimized) settings provides the same image quality but the optimized format provides a 2%-8% reduction of the image file size, so there is no reason for not use always this setting when not using the progressive format. The progressive format also provides the same quality, but offers an alternate method of displaying the image while it is downloaded, at the cost of a slightly increase in the file size (more details about progressive encoding).
• Advanced options
  Apart from that options, some programs like Corel PhotoPaint, Paint Shop Pro and IrfanView (this one is freeware) allow choosing the type of image subsampling, also called downsampling. The subsampling ratios according to the spec are 4:4:4 (no subsampling), 4:2:2 (standard subsampling) and 4:1:1. Adobe Photoshop also contemplates this option but it's adjusted automatically depending on the quality level selected: it applies subsampling for Medium and Low levels and uses no subsampling for Maximum and High (more details about subsampling/downsampling).
  
  While the standard subsampling is usually adequate for most images, providing an adequate ratio between image quality and file size, there are some situations in which using no subsampling (4:4:4) provides a perceptible increase in the image quality, even if you use a higher compression ratio in order to maintain the file size. There most notable cases are when the image contains some portions with fine details, like text over an uniform background, and images that contain almost-flat colors.