rss image import size max? Autoblog

I seem to be having an issue with the size of imported featured image that will post as thumbnail from my feed.
I'm using this feed http://data.nbcnews.com/drone/api/query/NBC/nbcnews/1.0/topstories and any images larger than 800x600 don't seem to be transferred to the post even though there is an image attached to the post? I'm lost on this... is there a max size an image can be?