How can I resize the image thumbnails to a particular size on entire store ?

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I have images of two different sizes in my store. When these images appear adjecent to eachother on product page, the two different sizes are displayed. How can I resize all thumbnails to same size? Refer image1 below for better understanding. Same thing goes for banner image. I cannot drag the image to set its position for proper view resulting the top portion of the image gets cropped on store. Image2

Image1