Since STK 1.2 the teasers pointing to pages retrieve the teaser image from the first image in the content.
When a teaser refers to an image gallery, no image is retrieved.
I guess because the images shown in the image gallery is not in the gallery's content (in DMS).

I think this looks quite bad, when all other referd pages a image is retrieved, but from IMAGE gallery not

See:
http://demopublic.magnolia-cms.com/demo-project/multimedia.html#main

I think it makes it very hard to have a consistent behavior on a site, now that the images used for a teaser come automatically from the content of a page.
With the loss of the definition on page-info level, the only way to manage the used image for the teaser on the target page is using a image within the page header.
The definition on the 'teaser overwrite' is not really a possibility, if you want top make sure, that all teasers pointing to a page display the same image. And for all automatic selection of an target page, it's not possible to define the used image on the content of the teaser, it must be defined on the target page. For example for the myMoneyWorks I created a teaser pointing to a section, and just randomly a child page is used wothn the teaser. This implementation would be much more complicated now, and especially much more confusing for the author.