You can run Tiger, Leopard, and Snow Leopard. You cannot run Lion.
No. An iMac is a computer from Apple. Mac OS X is an Operating System for Mac computers. Every Mac comes with a copy of Mac OS X - pre installed.
There are apps that will only work with Mac OS X which can be on any Mac computer - iMac, MacBook, Mac Mini etc.
Mac OS X 10.6 (Snow Leopard) only works with Macs with an Intel processor. The last version that will work with a G5 Mac is 10.5 (Leopard).
There are no viruses affecting Mac OS X.
The iMac G4 is able to run any version of Mac OS X up to and including, version 10.5 (Leopard).
Holding down the Option key (alt) while the Mac is starting up will offer you the choice of how you want to boot the Mac. Selecting Mac OS X will make that the default boot.
To upgrade beyond Mac OS X 10.4 (Tiger) you will need to purchase Mac OS X 10.5 (Leopard). If the iMac has an Intel processor you can jump straight to Mac OS X 10.6 (Snow Leopard) but if it has the older G4 or G5 processor then 10.5 will be as far as it can go.
MS Picture It is not available for Mac OS Leopard. To use it on an iMac you will need to install and run the Windows operating system.
No. You can only play games made for Mac OS on an iMac.
Yes, you can upgrade to Lion from any iMac that uses the Intel processor. But the newest OS is Mountain Lion.
You can not legally run Mac OS on any computer not designed by apple.
The G5 came with the OS 9 operating system and had a Power PC processor. It was the last edition of Mac OS. The iMac switched to the OSx operating system with the next iMac. There was an education only model G5 that did ship with OSx Panther.