You use the Save For Web & Devices dialog box (File > Save For Web & Devices) to select optimization options and preview optimized artwork.
If the entire artwork is not visible in the Save For Web & Devices dialog box, you can use the Hand tool to bring another area into view. Use the Zoom tool to magnify or reduce the view.
The annotation area below each image in the Save For Web & Devices dialog box provides optimization information. The annotation for the original image shows the file name and file size. The annotation for the optimized image shows the current optimization options, the size of the optimized file, and the estimated download time using the selected modem speed. You can choose a modem speed in the Preview pop‑up menu.
If an image contains more colors than the monitor can display, the browser will dither, or approximate, the colors that it cannot display by blending colors that it can.