hey there, you're just a bit ahead of Attract-Mode development on this one... 1.5.1 and earlier don't have an artwork scraper, there is only a fairly rough feature for grabing some game info from thegamesdb.net.
This is fixed in the next version though, which will have a (hopefully) decent artwork scraper. I am just in the process of some final testing and polishing things off for a new release, which should be ready very soon.