There is no easy way to achieve what you want. The only reliable solution is make sure all your product pictures are in the same aspect ratio (1:1, 3:4 or 16:9 etc..), they don't have to be in the same resolution, just the aspect ratio equality is enough. So, one of your pictures can be 750x750 while the other one is 600x600, as long as their ratios are the same, their heights and widths would be the same on the category page (because they are being scaled-down in the category pages), thus your website would look much more professional.
If you don't have too many pictures/products, i definitely recommend you to re-create your product pictures using a photo editor (photoshop or similar). Pick a ratio first that works the best for you. You can pick 1:1 (amazon uses 1:1) or you can pick 3:4 (which also works most of the time)
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