[bono]

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 slight increase in the file size

http://www.ampsoft.net/webdesign-l/j...mpression.html

* Baseline (Standard) - This JPEG format is recognized by all Web browsers.
* Baseline Optimized - This JPEG format option provides optimized color and slightly better compression. It is supported by all modern browsers, but was not supported in the very earliest of Web browsers. It's your best choice for JPEG files today.
* Progressive - Creates a JPEG file that displays gradually as it downloads, starting out blocky, and gradually getting clearer as it downloads. It doesn't make the image download any faster, but it can give the illusion of speed since the blocky image is loaded right away on a slow connection. With the majority of internet users on high speed connections today, Progressive JPEG is rarely used.

http://graphicssoft.about.com/od/gra.../f/summary.htm