the thing is you are placing images with different aspect ratios in a grid. They will always scale propotionally in the gridder.
If you want stuff to really look a certain way I’d recommend you play around with a layout software first. (sketch is pretty cool but anything works that enables you to use layout-grids)
That’s the easiest way to determine the desired aspect ratios and to prepare all your assets perfectly for the planned layout of your site.
Although it’s possible to do layouts in lay theme I’d recommend to do it the way I described above – Makes things way easier.