For example the ABC news feed includes a range of alternative image sizes including one selected as default via (isDefault). Other readers I’ve used select just one, but newsblur renders all 5 images one on top of the other.
The RSS feeds seem to be correct based on the media spec - all the pictures are the same just differing by resolution. As the parent says all the pictures are shown one after the other.
media:group is a sub-element of item. It allows grouping of media:content elements that are effectively the same content, yet different representations. For instance: the same song recorded in both the WAV and MP3 format. It’s an optional element that must only be used for this purpose.
isDefault determines if this is the default object that should be used for the media:group. There should only be one default object per media:group>. It is an optional attribute.