There is a much needed improvement for mosaico extension to display quality images specially on regular smartphones.
Images uploaded in 2 columns (e.g 258 x 100) or 3 (e.g 166 x 90) columns blocks when upscaled to render on mobile devices, ends up with stretched out and lower resolution image.
We intend to solve the problem by providing config and processing where images could be uploaded to 2x (or 3x) of their size specially for 2 and 3 column layouts. The solution could also work for single column images.
See this thread for more details on the requirement and spec - https://github.com/veda-consulting/uk.co.vedaconsulting.mosaico/issues/50.