I set up the size of preview pictures to 250×150 pixels. I did not tick the box below to resize to the exact values as I want the height relative to the width (usually 250×86).
It worked perfectly until I noticed that new posts are displayed with preview pictures of exactly 250×150 pixel. Somehow, I do not know how, the above mentioned box was ticked. I unticked the box but the theme does not resize automatically. Is there a cache to be emptied or how to I resize?