How do I restore a disk image on the startup disk?

Hello,

Where do I begin? Ok, my computer is an iMac and I mainly use OS 10.6.8. I have partitioned the disk into three parts: Snow Leopard, Lion and Tiger ( and I'm thinking of deleting Tiger). The main OS I use on my computer is Snow Leopard (10.6.8). I wanted to try out Yosemite so I upgraded the Lion partition and it soon became Yosemite. I was able to switch back and fort a couple of times last night, but today, when I went to the Startup Disk to go to Yosemite the disk image for Yosemite was gone and it only shows the disk image for Snow Leopard. I did a little research and if I'm not mistaken and read correctly, it indicates that Apple has done away with the disk image for Yosemite? How is one to switch back and forth between systems? Thank you for any insight to my question. -Della

P.S. If I have posted in the wrong category, please feel free to repost in the correct category, but please let me know. Thank you!