Yep, so in that case the designer should be able to provide you with:
a. All images of the same aspect ratio eg. a square image.
b. All images at a minimum resolution, i would suggest a minimum of 768px
So if was creating those images, i would start with a square canvas eg. 1200px x 1200px ( or whatever maximum size u may use them for ) which is the same for all images, and make my image fit that space. Then Export the image at the size you need for your webpage
This way you DO NOT mess with the width / height settings on the Image Block and then there won’t be the distortion or pixelation you see.