Go to Image, Image Size. Make sure "Constrain Proportions" is checked.
Under Document Size, change the measurement increment to Inches. Select a Width of 8 inches, and allow the Height to be calculated automatically.
Set the Resolution to 72dpi - while higher resolutions are useful when printing, most web sites won't display any advantage from a high resolution.
Click OK to confirm your changes, and save your file. Your reduced file size will depend on the original size, but these settings will normally produce a file size that can be uploaded to the Layout Gallery.