Go to Image, Resize/Resample. Check "Set new size", and select inches under Units. Select a Width of 8 inches, and allow the Height to be calculated automatically.
Check "Preserve aspect ratio", and change the DPI to 72 - 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.