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.

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.