So... i found this topic searching for the same question paffley did... and while the solution for him was to use the "marquee" category, I wanted instead to keep marquees showing, and displaying screenshot and boxart on my theme's video view for the games that doesn't have video. So, I was in need of a third set of displaying artwork... and after searching a lot, I found it on the development fork of EmulationStation!
For those that, as me, ended up here, here it goes the "workaround":
you have to install "emulationstation-dev" instead of the default "emulationstation" for it;
then you need to change the resource (probably a repeated "image" field on the same position and size of the "video" field) on your theme to the "thumbnail" tag (i used ComicBook theme for this example, on the video view);
lastly, after scraping (i used skyscraper), on gamelist.xml, change your boxart XML tag to "image" and your screenshot XML tag to "thumbnail" (i did a replacing macro on notepad++).
Tested here and works flawlessly! You don't have to switch exactly the same XML tags, it's only an example of how I did mine.