Ionic Avatar List - How to keep image's proportions?

Hello,

I’m using ionic list with avatars as documented here http://ionicframework.com/docs/components/#item-avatars

My problem here is, that my avatar images are not necessarily squarish, so if an image has a lot more width than height (or the other way round), the avatar gets stretched and it looks pretty bad.

I was unable to fix this using CSS. Anyone got a solution?