Transparency error in Pre-flight?
I'm preparing to send a illustrated middle grade reader to print (Ingram Spark). However, I did a pre-flight and for the first time selected 'Images' in the check panel.
Now every image in the book is being flagged with this error message:
Problem: Object has transparency applied
Fix: Edit the source file to remove all transparency.
All the illustrations in the book are simple black line drawings done in Photoshop. (examples below) All the pictures where placed into my CC library and then placed into Indesign from there. Several of them have layers, but they don't have any effects such as drop shadow or glow.
Things I've tried from forum trawling so far:
Flattening the image in PS
Adding a white background layer in PS and naming it 'Background'.
Flattening the image in PS with a white background.
Checking the transparency in Indesign (nothing to tweak it's all set to opaque).
I'm quite new to indesign, so there's obviously something conceptual regarding this transparency issue that I'm not grasping, hence why I'm finding it so hard to solve.
However - do I even need to address this issue? IngramSpark states that all they need for the interior content is 'a print ready pdf' preferably 'in PDF/X-1 format.'
Possible to ignore and just export as pdf from Indesign?
Any thoughts or help appreciated thanks
Example of pictures in book (from indesign) and how the layers look (from Photoshop)

