Usually when merchants ask this question, what they really mean is “how do I make my thumbnail images the same size”. All Shopify themes dynamically generate a smaller version of your images to create a thumbnail, so the goal should not be to make the size/resolution of the images the same. Generally, the larger the image is, the better the quality will be. Therefore what you really need to modify is the aspect ratio (the length/width ratio).
How should I standardize the aspect ratio of my images?
Which tool you should use depends first of all on what size thumbnails your theme is using. Please visit one of the following two guides:
If you thumbnails are square size, please click here.
If you thumbnails are taller than a square, please click here.