Email design should correspond with the website or the corporate style to make it easier for a recipient to recognize you.
An email is not a souvenir photo, you do not need a high quality picture here. That is why you should optimize pictures for 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
Again, watch the size of the GIF to be no more than 1 Mb, otherwise no one will appreciate you efforts because the picture will just not download. Sometimes when you open a promo email 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 through that you will see it on a Retina display device. For the layout not to be blurry we double its size, making the content width not 600px but 1200 and the text not 16px but 32.