An email is not a souvenir photo, so you do not need a high-quality picture here. That is why you should optimize pictures for the web, otherwise, they will be heavy and will not display. To reduce image size we use CompressPNG
(you can also compress jpg or pdf there) or IloveIMG
Sometimes, if you open a newsletter on your iPhone or Mac, the image is a little blurry. It does not mean that the pictures are of a low quality; the designer just didn't think
that you will see it on a Retina display device. To avoid such situations, you can double the size of email content: make the width of pictures 1200px instead of 600px, and the width of the text 32px instead of 15px.