Select your layout, and click on the Edit Picture button. From the Edit screen, click on Save As at the bottom right.
From the Resize drop-down at the bottom, choose Best for Web. Click on Save. Choose Yes to overwrite the current file, or No to return and enter a new filename.
Close the Edit window. 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.