It looks like creating a slide show in Impact that has a collection of mixed aspect ratio images - basically a mixture of portrait and landscape images - really does not work. In fact, using portrait images doesn’t work well at all - even if all the images are portrait orientation. Is that the experience of other users also? Comments would be appreciated.
That is not what the stack was made to do. It was made to be like a hero header where it spans the whole image across the top. There are other settings, but they all have to be the same orientation.
Image scaling is dictated by physical geometry and not any particular stack.
If you set impact to a portrait aspect ratio then it will of course display the majority of the image, in fact all of it if the image aspect ratio is the same as the impact setting.
If you are trying to display a portrait image in a landscape container or vice versa then it is a physical inevitability that not all of the image will be displayed.
In order for an image to cover a given container, the minor axis (i,e whichever is the smaller) must be scaled to the corresponding width or height of the container. In order to preserve the aspect ratio and not distort the image, the other axis must be scaled by the same factor.
This will, for a portrait image in a landscape container, make it considerably taller than the container and so a large proportion of the vertical axis will be out of view. This is, in essence, no different at all than an old fashioned photo enlarger.
The point of Impact is that it is a proportionally sized container - i.e. it does not continuously expand and contract vertically to accommodate its images, rather it scales the images to fit that proportion. It would actually look pretty ugly if it were continuously changing size during transitions and pushing page content the up and down.
Mixed aspect ratio images will pose similar problems for any container, be it a stack or a physical picture frame on a wall. Careful cropping of images can help but if they are required to be vastly different then maybe a better approach would be to use a slide show that does not cover its container but rather just allows white space around the sides.