You can use Edit in Photoshop Option from the Lightroom to change the Background for those two Images.
Select the Image
Click on the Edit menu
Click on Edit in Photoshop
Images will open in the Photoshop.
once the Background changed to white
Click on File menu
Click on Save.
You will find the edited copy for the Image in Lightroom.
For changing the Background you can follow the below tutorial.
Let us know if it helps.
Thanks for your time in your detailed reply, I appreciate it.
Just clarify my query, I do not want to change the 2 portrait images against a black backdrop, I want them like that.
In the Lightroom book module, my query relates to printing the book. I am designing an 84 page book. With this book, can you please confirm Blurb is able to print a book with 82 of pages in black background color and just two pages (which are the 2 portrait images with a black backdrop) in white background color
- Apply black background to all pages
- Select individual page (outline becomes yellow)
- Uncheck “Apply background globally”
- Change background color to white which will only be applied to selected page