Ok, the code I showed you only works with the native summary field. It will not work the the TV fields. Future versions of such snippets will most likely support TV fields
So is the problem with MODx or NewsListing snippet? I'm sure that richtext summary field is a HOT topic :!: Of course you may insert an image html-code in a plain-text field, but it is not comfortable at all :? .
The NewsListing snippet would have to be coded to recognize the summary tv field