How do you access photos once you backup your iphoto library to an external harddrive. Want to be able to access specific photos versus having to open the entire library in iphoto every time

You cannot access individual images in a library without opening the Library.

Back ups: Use an app that will do incremental back ups. This is a very good way to work. The first time you run the back up the app will make a complete copy of the Library. Thereafter it will update the back up with the changes you have made. That makes subsequent back ups much faster. Many of these apps also have scheduling capabilities: So set it up and it will do the back up automatically.

Example of such apps: Chronosync - but there are many others. Search on the App Store

Time Machine will also do incremental backups.