I can't use the iPhoto library as desktop backgrounds on Mavericks
I solved my problem by using iPhoto's Photo Library First Aid and repairing the permissions of the iPhoto library.

To activate the Photo Library First Aid

Hold down Option+ Command  while starting iPhoto.

A Window with several options will appear.

To repair the permissions check Repair Permissions and click the Repair button



If that does not solve the problem, again activate Photo Library First Aid and try the next repair option, "Rebuild Thumbnails." Continue through repair options until problem is solved. (In my case, "repair permissions" was not sufficient, but "rebuild thumbnails" solved the problem, so that "Photo Library" appeared in selection list for screen saver.)