Trying to install operating system but it keeps spitting out disk.

I have the iMac M5521 with a slot loading optical drive and after 3-5 seconds after I insert an operating system disk, it ejects it. I tried holding down the option key to select a disk to boot off of but noting shows up. If i let it power up on it's own, it will show a finder icon on a blue folder. I'm assuming that means their is no operating system. I tried several disks. I got the operating systems off of pirate bay (a torrent sight). One of the disks is 10.4 tiger.