Mone, I've got a solution for you! It requires a couple extra steps on your part, but if it's important to you to see the full 2-page layout spread in your thumbnails, it's an option.
Here's how a 2-page layout I created shows up in my gallery:
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So I click on the layout, and it brings up the detail view:
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On the right side on the screen, you're looking for the row of buttons that begins with Watch. On the right end of that row, click on the drop-down button with the three dots, and the "More Options" pull-down menu appears. Click
Change thumbnail.
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The system gives you two options: Use default thumbnail, or
Use custom thumbnail. Change it to custom, and then upload an image of your 2-page spread that's square with a white background at the top and bottom.*
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Then click
Save.
When you back out to your gallery, your custom thumbnail now appears!
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* I know it's an extra step to save your layout thumbnail... The quick and easy way to do this in Photoshop is - with the JPG of your 2-page spread that you've uploaded to the gallery open - navigate to
Image ... Canvas Size. Change the number in
Height to the same as the number in
Width. Make sure the
Anchor field is set to the center of the layout, and that
Canvas Extension Color is set to white.
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Click
OK, and your newly-square 2-page layout will appear. Then just
Save for Web like you normally would.
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I hope this information helps!