Image sizing in Email designer

I had this image that was 500 x 300 px. And it looked fine in normal desktop-mode inside an email that was 600px wide in total.

In the portrait / phone -mode however it looked really square and boxy. Here it was adjusted to 480 x 300 px.

I'd love to hear others users' best ideas when it comes to image-sizes inside Emails designed in the email designer?

My questions:

• Do you ever use images that are taller than 250 px in emails? Because the get very square in portait / mobile -mode.
• Is there a function to keep the width:height ratio when adapting images to portait / phone size?

Idea:

And lastly, I was just thinking about that there are more and more high resolution devices used. Will Email designer start using em or % instead of px in the future?