There are two common causes for an incorrect image aspect ratio:
To help avoid these issues, consider using an image CDN, to help automate the process of creating different size versions of your image.
Also check any CSS that can affects the image's aspect ratio - if you're having trouble finding the CSS that's causing the incorrect aspect ratio, Chrome DevTools can show you the CSS declarations that affect a given image.
Finally, check the image's width and height attributes in the HTML. When possible, it's good practice to specify each image's width and height attributes in your HTML so that the browser can allocate space for the image. This approach helps to ensure that content below the image doesn't shift once the image is loaded.