The Debian installation guide
http://www.us.debian.org/releases/stable/i386/ explains the various different options for installing, including from a hard drive, but if you have simply downloaded the images,
I'm pretty sure there is no way you can boot a .iso from your hard drive, as the system cannot read what the image contains. If you open the .iso using something like isobuster and have is stored somewhere on your hard drive that will be accessible during the install, following the instructions to boot from floppy or an exisiting Linux install might be of some use, but I haven't tried a hard drive install.