Sorry for late replay, I just haven't had the time to give you detailed answer.
OK, so if we take a look at these two images from reference site:

You can see that pictures have different ratio (height/weight) so using any available method from Admiror Gallery would never have this kind of effect.
If we use None, we would get cropped images like this:

If we use Width:

If we use Height:

Note that when you change Auto Size parameter, thumbnail images won't be recreated. Only changing Thumbnail Width or
Thumbnail Height will make AdmirorGallery recreate the thumb images.
In my opinion (that could be wrong

Width is the most appropriate to what you want to accomplish, along with some CSS changes to create border around the thumbnails and center them.
If none of these cases are not OK for you, is it possible to post a screenshot of the expected behavior using these two images as example (you can download them from reference site)?