Images Not Displaying Properly on Blog Page or Single Page

Hi There
My posts have a featured image that is 600 x 250. I cannot get the image to display properly on the blog page, it gets spread AND cropped (weird).

I have set the ‘Featured Image’ on my single-page to be 250px in height, but again the images displays incorrectly. The image is cropped top and bottom (despite being exactly 250px tall)