iPhoto 06: Remove lower-res duplicates?

Since I've finally upgraded the HDD on my Macbook I now have space to import the photos from our wedding a few years ago. They were all put on CD from Wolf Camera, and when I imported them I got a high-res original and a lower-res duplicate of each. Within iPhoto there is the full-size "roll" and then a "roll" for each labeled "low-res". As an iPhoto novice, how do I safely delete the ones listed as "low-res" without compromising or corrupting the whole library?

Thanks for any info!