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, 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.