Ar this is a regular problem i get when people try to install Windows XP or Linux on Boot camp e.t.c.
What's happened is Linux has Tampered with the boot up files and now it won't be able to boot into Linux or Leopard.
What you need to do is insert you Leopard Install disk that came with you iMac Into the Disk Drive, the disk/disks are grey and may be two CDs or 1 DVD.
Restart the iMac and the moment you hear the chimes go off when it boots up, Hold down C on the keyboard.
The boot disk for leopard will then boot up,
From here follow the instructions and if your lucky you may be able to upgrade, if not, go through the system utility and repair disk permissions e.t.c.
If nothing works, You will have to do a clean install,
But you may be able to recover a thew images and mp3 files with a great bit of software i like to use,
http://www.prosofteng.com/products/data_rescue.php
You do have to pay for it, and you will need to install it on macbook e.t.c. if you have one in order for it to work
If your confused there is a number of guides out there on that website or on google
Hope i have been of any help
Regards