Is the white set to overprint in the images? Should not be.
What file type are those images?
I am not sure what you mean by overprint? And are you talking about the white frames that are disappear or just the white background behind all of the cards? The ones that are disappearing are .png files.
Overpint property is a setting which is applied either in Illustrator or in InDesign.
I am talking about the white things. You have scaled down your screen shot so it is impossible to see what you mean. Don't scale images when showing here in the forum. Why? They will be scaled automatically, but if they have original size, we would be able to zoom in.
Why are you using PNG? What are the original files? If they come from a graphic application like Illustrator use AI or PDF/X-4, if they come from Photoshop use PSDs.
What is the purpose later?
Don't use Apple Preview for opening PDFs! Don't do it!
Apple Preview cannot show correctly all flairs of PDFs available. Use Adobe Acrobat only! Apple Preview is not more than a wast of space on your disk, not usable at all.