Product Image different sizes

Hi, I’ve a problem with how the images are shown on my site.
All the images that I upload on my site are showed squared, it doesn’t matter the image format or size, the result is always the same. However, all the images from the Dummy Content are shown larger and not squared. It happens in both thumbs and the product page itself.

I just want that my images look like the Dummy’s, even if I’ve to use the same image format.

Thank you for your time!