Featured Image Cropped Out of Shape

All of my featured images are cropped out of shape. The dimensions of the original image are 3051 * 2123, but when displayed as the featured image of a post, it is cropped to 1920 * 480.

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Original image:
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I realize that 1920 * 480 is the default size for featured images. However, when I use some of the stock pictures (came with the Poseidon theme) as featured images, the theme doesn’t crop the pictures and display them at their respective sizes.

How do I fix this and show my featured images at their own natural size?
Thank you so much for your help!!