"selective" jpg... does it exist?

To use masks to modify JPEG quality:

   1. In the Optimize panel/palette, choose a JPEG setting from the Settings menu, or choose JPEG from the file format menu.
   2. Click the mask button mask button to the right of the Quality text box.
   3. Choose which masks to use:
          * All Text Layers to use the masks from all text layers in the image.
          * All Vector Shape Layers to use the masks from all shape layers in the image.
          * Channel to choose an alpha channel from the menu. In ImageReady, you can choose Save Selection to create a new alpha channel based on the current selection.
   4. To preview the results of the weighted optimization, select the Preview option.
   5. Define the quality range:
          * To set the highest level of quality, drag the right (white) tab on the slider, enter a value in the Maximum text box, or use the arrows to change the current value.
          * To set the lowest level of quality, drag the left (black) tab on the slider, enter a value in the Minimum text box, or use the arrows to change the current value. 
   6. Click OK.