You don't cannot Preview, it is an app integrated into OS X. However you can set the prefererence to open PDF's with either Adobe or Iphoto. You have to choose one or the other, you cannot choose both. You can also right click the file and select Open With and then choose the app you want to use.
It's worth pointing out that if you set your default app for "pictures" to iPhoto, then every time you double-click on a JPEG or TIFF to view it, that file will get added to iPhoto's Library.