Open your image. Go to File, Save As, and save a duplicate of your layout. For example, if your original file is bluepage.jpg, you may choose to use something like bluepage-sm.jpg. This will allow you to reopen the original if your results aren't satisfactory.

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.