Thumbnail/Picture discrepancies and scrolling wonkiness

I scanned a bunch of old pictures, added them to iphoto, and now there is a strange discrepancy between the thumbnails and the photos. Depending on the size of the thumbnail (controlled by the zoom in the lower left corner) the thumbnail either doesn't match the picture at all (when the thumbnails are at the smallest size) or it matches until you scroll, and as you scroll the new pictures coming onto the screen have the wrong thumbnail before resolving into the correct one. It makes scrolling through pictures to search for images impossible as you aren't seeing the pictures, merely thumbnails of other pictures until you stop, when they all change back to the correct thumbnail (at the larger zoom sizes).

I am using a 27", Late 2009 iMac (2.8 GHz Intel Core i7) with 16 GB DDR3 Ram and an ATI Radeon HD 4850 512MB video card. I am using iPhoto 9.6. However, I doubt it is machine related as there was no issue before adding the scanned pictures, and all of the digital pictures I have do not experience the same issue.

I have deleted the plist for iPhoto with no change in symptoms.

My question - is there a way to "rebuild" the thumbnails, or do you know anyway to prevent this craziness?