Image scaling not working properly

I added an image to the Media gallery that was greater than 500px height, scaled it down to 150px (the size our template theme category page expects the featured image to be), attached it to the post as the featured image, but it shows up cropped on the category page, and when I right-click and open image in new tab, the pic is the right size but cropped! If I look at it in the Media gallery, it shows the full picture at the correct scale… how do I fix this?