Low image quality in iPad (retina display)

Hi,
when i check my project on an iPad with retina display the images in the element “image gallery” in the detail page of Portfolio section have low quality. When i click and enlarge them, the quality is ok. Why this happens? Where can i set the quality of the image thumbnails? Is there a plugin to resolve this trouble?
Here the link for exemple: https://zecchin.maso.it/?portfolio-post=appartamento-privato-oderzo
Thank you for your replay.

Elena