TL;DR The images show pixelated.

Every modern Mac has a HiDPI (a.k.a. Retina) screen, where each 
point (logical pixel) maps to 4 physical pixels, a.k.a. 2× scale. 
I in the attached small screenshot, please compare the sharpness 
of the image with the text next to it. Emacs renders the image at 
1× and the text at 2×.