Photoshop Mockup Placed Image Pixelating & Losing Quality

[ToreyTryon]

I'm creating a product mockup in Photoshop using a prepurchased mockup generator and once I place my image into the mockup it loses clarity after zooming beyond 100% although the original image maintains its clarity. The original was created in InDesign. I've tried copying and pasting into Illustrator first and then using that image (tried exporting at as various file types) and am still having the same problem. What do I need to do to maintain the quality of the original image so that when it is placed it remains crystal clear?

[MichelBParis]

It looks like the original is severely resampled when downsized to fit in the place holder. With less pixels, the image can't be as detailed as the original.