Here are images from a McCain ad that first appeared in June, way before Obama ever mentioned anything about his face on a dollar bill:
The ad says:
Let's look at what Barack Obama has changed so far. ... What will he change next? The Statue of Liberty? Mount Rushmore? The $100 bill?
I'm not sure what having his face on the Statue of Liberty is supposed to convey, but to the bigger point, what do they mean about changing the $100 dollar bill? In what way would it "change" if he is elected and distinguishes himself to the point where he would be considered for that honor (change that is different or more worrisome than calls from conservatives to put Reagan's image on coins/currency)? Whatever they mean, Barack Obama did not raise this issue first (contrary to the impression left by this editorial in the Washington Post, and by other reports on this issue).
Update: Jonah Gelbach at Economists for Obama points to the choice of the still image on the ad linked above:
Bottom Decking Started in June, by Jonah B. Gelbach, Economists for Obama: ...Jed Lewison ... notes that
As you can see, the video's still image is of Obama on currency. Note that the image doesn't occur until about three-quarters of the way through the video. The way YouTube works, the default image occurs at the midpoint. In other words, McCain's campaign affirmatively [chose] this image.
No doubt all this will be well covered by the MSM...