Fixing Stretched Images
Regenerate Thumbnails allows you to regenerate the thumbnails for your Featured Images. This is very handy if you’ve changed to a theme with different featured post image dimensions.
Is your WordPress theme not working how you want it? WPZOOM themes don’t experience problems with stretched images, plus they’re easily customizable, well supported and responsive. Switch to a WordPress theme that works.
If you need more options or to regenerate only specific image sizes, check out Simple Image Sizes plugin.