For a local App that will be only available on our own tenant (so no publishing to the MS app store), I want to display an image / image carrousel on the app's "splash screen", just like the image below:
The image markdown seems to be supported, however when I add the image markdown, like so:

It does not show the image, but the alternative text and a link to the image:
How can I display images like the example?
This is not part of the manifest. You can set this while submitting your app to the app store. Please reach out to teamsubm@microsoft.com for any further queries on this.