Need a bit more info
Do you hard code your preview text into your emails or does your esp do it?
What encoding are you setting in the head of your document?
Depends on your audience.
Retina images tend to be heavy, so be weary if a large majority of your audience views your emails via mobile. I for one wouldn't be happy if I found out some vendor was sending me huge images in the background of their emails.
a google search found this tool from campaign monitor: https://backgrounds.cm/