for my industry I always look at the whitepaper from Silverpop:
You can also find the one of 2014 and perhaps of 2013.
Ciao sunsets a. I work in the fashion industry (therefore with the purpose of Retail) and our email marketing strategy is based in a large part about the use of images, due of course of the emotional factor. We always made a particular attention to the graphic optimization, but what I can say is that in comparison with at least 2-3 years ago, we make also more attention to the compatibility. The fact is in many case it's more difficult than it seems, explain to the Brand Managers that the reading of our emails is at least important as the maintainance of the Brand identity and that to follow the trend (and the best practices) does not mean to give up the second one.
Technically, as Lloyd wrote, there are companies that sell services like the one in example, and in fact our ESP has an option of that kind, that has a cost we're glad - and that we have - to pay ;-). I think it's not a matter of "can or can't", but a matter of sensibility.
I was used to insert an "invisible" text of #color as the same of the background, in order to see it in the mobile preview, but not in the mail while it is open. I have to say that this way is a lot faster, although in this case when a recipient forwards the mail to another contact, the Preheader text will be written in the mail itself. Thank you!
Generally I don't specify the dimensions in the style and only with the normal attributes, the retina images are good also in Outlook.
Cappuccino or orange juice to start. Music is my Radar especially in the headphones when under stress situations. Sometimes, on the very edge of a deadline, The Sound of Silence mixed up with a cup of tea, some minutes of fresh air and the direct light of the desk lamp, are equally precious.