Hi Gorast,
You need to use same resolution pictures, or make sure they have same aspect ratio.
So if one of your product image has 900x1500 (which is ratio of 3:5)
Make sure your other pictures uses the same aspect ratio, something like 600x900 or 300x500
So thumbnails will look the same when they are changed in the product details page. You can arrange the aspect ratios by adding pure white to top and below of them. Since your website background is also white, there won't be any odd appearing. If you browse some well-known ecommerce websites, you would instantly notice that all the product pictures they use have EXACTLY same resolutions or same aspect ratios, because only this way everything look much natural and cool.
Regards.
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