Sometimes, you’ll add a featured image, or a graphic for a slider, and notice that the image is either squeezed or stretched when it’s displayed on the front end. That’s almost certainly due to settings in the theme that you’ve selected.
Contact the theme developer and find out the proper dimensions for images used with the theme. You might find that all featured images are required to be 640px by 360px or that images in sliders must be square. Once you’ve learned about the proper sizes, you can use your favorite image editing software to trim your graphics accordingly.