how reduce the size of all photos in my iPhoto library? My iPhoto libary is 58 Gigs.  They are all snapshots, nothing professional.  Most of the photos were taken at 12megapixels because that's the default setting on the camera.

I'm sure they will all look fine if they were reduced to 5megapixels.

Is there a way in iPhoto to reduce the size of all photos in the Library to smaller?
Before you take this (drastic) step, consider a couple of things. 


Make sure the trash in iPhoto has been emptied -- not everyone realises this needs to be done, and it can take gigs of space.
iPhoto creates a duplicate image when it rotates an image, and the original is not required. I'm not sure if there's an easy manual way to remove these, but I use CleanMyMac (http://macpaw.com/cleanmymac). It has a free trial and it will save a large amount of space. 
If you're worried about performance due to the size of the library, consider splitting it up helped me. I got to about 60gb, and it was very slow, so I used iPhoto Library Manager (http://www.fatcatsoftware.com/iplm/) to split it (I split into 1 library per year)