no i have not downloaded it i have the disc!
And Leopard isn't available for purchase as a .dmg file.
The only way you can have that sort of file is by doing one of two things:
A. Downloading it illegally from a torrent/P2P site
B. Using an extracted .dmg file from the actual disc... which is pointless to use in this case if you actually own the original disc.
If you don't own the original disc, then the .dmg is likely a pirated copy obtained by means other than downloading and therefore an illegitimate copy.
You will need to get a physical, retail disc for Leopard to solve your issue.
You can purchase it from any reputable retailer.
If you had a disc and lost it, you can contact Apple and they would be able to furnish you with a replacement disc for a fee.