Steps to Reproduce

https://zkfiddle.org/sample/2jhdm5m/2-image-distortion-issue
open the menupopup by clicking "File"

Current Result

the 200*100 image is forced into 20*20
and the 100*300 image scaling based on the ratio of width 20*60

Expected Result

Proportional zoom the image if posible
and check the spec: we want that the picture be able to grow bigger to make menuitem-content bigger or not

Debug Information

we have these styles:
.z-menuitem-image 

{ min-width: 20px; min-height: 20px; max-width: 20px;}

Workaround

if we don't want to make menuitem-content grow bigger:
.z-menupopup .z-menuitem-image 

{ min-width: unset; min-height: unset; max-width: 20px; max-height: 20px;}

or we just keep the width:20px for the text aligment and let the height grow bigger:

.z-menupopup .z-menuitem-image 

{ width: 20px; min-height: unset;}