Poor resolution of thumbnails

I’m using an Image Display object in a matrix to output 16 jpegs to an A4 page. The jpegs are all at least 1000 x 1000px in size and 72 x 72 resolution and each is crystal clear at when opened in Photo or Preview, but they are blurred and pixelated when printed in color at best resolution or output as PDFs at 133px square in the matrix in the Panorama form (b26).

I’ve tried reducing the jpeg sizes, and converting photos to other file formats like .heic, .png, .tiff and .pdf but these changes make no difference to the poor quality of the thumbnails.

Is there a way of improving output resolution by this kind of setup?

No, there is not.