Featured image size

The size of the featured image on my site used to adjust to the size of the actual image so it would display nicely and entirely on blog posts without being cropped or streched.

Somehow now all featured image are streched or cropped to the heights set in the highend options ‘blog settings’.

For example, if I set the height to 500px as it is right now, images that are longer get cropped liked http://socialvignerons.com/2015/08/15/how-to-say-wine-in-8-of-the-most-spoken-languages/, images that are shorter are streched vertically but cropped horizontally like http://socialvignerons.com/2015/08/14/wine-quote-by-christian-moueix-more-than-food-pairing/ .
So virtually no featured image displays nicely on the site.

I couldn’t find any option to allow the display size of the featured image to be automatically adjusted to the real size of the image and fit all nicely in the featured image section.

Not sure it’s related to the recent theme updates. Have just noticed it now but not sure where it originates from or when.

Thanks for your quick help on this

CHeers