I think you already did it in your second test? To create a uniform carousel, you will need images with identical aspect, and unless they are identical by nature, they need to be cropped.
An original image looks like this:
Top crop and resize the image, replace /content/
, where w640 represents the width and c.3.2 represents the CROP ratio. For example:
The above creates a 3:2 aspect image. You can choose any aspect, for example 1:1, and you can also change the width, for example:
So, for your carousels to be uniform, you need to include images that are cropped at the same aspect using links like above.