Just to expand on this a tad so the original poster doesn't get confused... it sounds like they are relatively novice when it comes to the Mac (and, perhaps, computers in general, but they didn't state one way or the other).
There are two reasons you can't use what you downloaded (in its current form) in order to install Leopard:
1) It's in "disk image" format, meaning you need to properly burn it to a CD-R or DVD-R in order to be able to boot from it, and...
2) It's illegal. There is no legal way to obtain Leopard other than purchasing a retail copy or using the Leopard restore CD/DVD that came with your computer, if one did at all. There are no other legal ways to obtain Leopard. Since you didn't use either of those two legal methods, we have our hands tied here and cannot help you -- it is against the forum rules to discuss software piracy related things, and unfortunately, that is exactly the category your actions fall into. It's not moral or personal in any fashion... you know the drill: rules is rules.
If you really want Leopard on your computer, then you, again, have two options:
1) Obtain help elsewhere. There are plenty of shady places on the internet that deal with what you are trying to do.
2) Obtain a legal copy of Leopard, which will not come on a burnable disk, but rather a nice, silkscreened CD or DVD, and install Leopard with that. If you encounter trouble with this method, we would be more than glad to assist you in anything with which you need assistance.