Well, what do the fMRI scans of people looking at fMRI scans look like compared to those looking at the same information without the fMRI scans?
But on a serious note (now that I've actually read the article) it's worth observing that the further back a study was conducted, the more likely it is the images had a real effect. According to the model of persuasion, pictures generated by a brand new technology are more likely to have persuasive effect because it is less likely that people will be have the knowledge about the new technology to evaluate evidence from it. As the technology becomes more familiar people can get more into the meat of what it is telling them and stop looking at the pretty pictures.
Thus a shift over time could be reasonably expected, and a study from 2008 might show more influence than a contemporary study even though nothing has changed about the methodology.