Those ISO files may not be reliable. As they are not official Apple installs, we don't know what unique configuration details they may have.
Apple did release 10.5.6 retail installer DVD that looks like this, and does not say Dropin, Addin, upgrade, or OEM:

This can be used univerisally on all 867 Mhz (1 Ghz is higher) and higher single CPU PowerPC Macs with a built-in DVD drive. Firewire DVD drives could boot this as well.
If you have an Intel Mac, you are better off getting 10.6.3 Snow Leopard retail that does not say Dropin, Addin, upgrade, or OEM, and that looks like:

The Mac must be older than March 15, 2010. Newer Macs are in a transition as far as capability of installing from CD or internet.
The prebundled disc on pre-July 22, 2011 Macs that are newer than March 15, 2010 are one of the few ways you can install Mac OS X on those Macs. Please ask if you believe you are in that transition period for Macs.