Your #each loops over the content of App.tweetsController (it is not the nicest way to address the controller this way, but that’s not the point of the question).
Within your getJSON callback, you loop over data.items, but you only receive 1 item because you set the limit to 1. Thus, you only call the line me.pushObject(...) (which itself is perfectly fine) only once, so you only have one item in the controller.
Let’s look at the extraction of the thumbnails:
$.each(this.thumbnails, function(){
result = this.image.url;
});
This only stores the last url found in result, if there is more then 1 thumbnail, the old one will be overwritten. I think you have to rethink the data structure you want to extract, use an Array to store the thumbnail urls and then another #each loop inside the article to loop over a post’s thumbnails.