Instead of using the Time Capsule or a drive with the Time Machine backup for the active iPhoto library, purchase a different drive, copy the photos to it, launch iPhoto wih the Option key held down, and point it there.
Having your backups on the same drive as the original data won't help when that drive fails.
There are two ways to use iPhoto - (1) when you add new pictures they are added to the iPhoto library which is in your Pictures folder or (2) they are transferred to a folder of your choosing (usually the Pictures folder). In either case, unless you told TimeMachine to ignore some folders when backing up, all your photos are on TimeMachine already.
If your pictures are in the iPhoto library you'll need to copy the entire iPhoto library, it isn't really possible to look into the library and see individual photos.