How do I install Linux on an Intel iMac with no OS nor CD drive What I want: 


Linux on an Intel iMac (Core Duo).


What I have:


An empty PC (17" Intel iMac) with broken CD drive. It's model A1173.
This PC with Ubuntu 12.04 and an old Windows Vista partition.
a USB flash drive and an Ubuntu ISO.


Problems:


No CD means the only boot drive I could use is USB.
There is no BIOS on Macs, so I can't set boot settings or even see if it detects my USB drive. When I start the machine and press Option, the first and only thing I see is an old corrupted Windows XP partition and not a single option or additional information.
So assuming blindly that the Mac hardware/firmware works normally, I don't have any Mac OS to use any of the tools that I found on different tutorials for building a bootable drive for Macs.
I can't find much software on Linux/Windows to substitute to those tools, for example among others converting an .iso file (Win/Linux) to .img (Mac I guess). Which makes me think that the scenario where someone like me has Mac hardware but no Mac OS is extremely rare.
So other than finding someone that has a Mac I have no solution. So I ask, what would you do? The only thing is it should not involve any money (I know Mac software is rarely free) which also excludes getting any Mac OS unless I can use a free macos.img for VM or restore the original Mac for free.
Looks like you have all you will need already, you were just missing the following bits of information.


You should checkout a Ubuntu instructions for Intel Macs, so no need to worry about a BIOS your running EFI. Ubuntu for Intel Macs and additionally the Ubuntu Intel Mac CommunityHelpPages looks to be loaded with lots of resources that you might find helpful.
Since your iMac is an intel one, you can use a standard Ubuntu ISO built for any x86 PC. However since your iMac is a Core Duo it will need a 32bit Ubuntu as noted on Ubuntu's Mac Community documentation. 



  ... If you have a Core2 Duo Mac (as oppossed to the Core Duo), it is capable of using the AMD64 (64bit) CD otherwise you have to use the standard version...


The standard version meaning a 32bit version.


Pick and download a 32bit version of Ubuntu , the Mac documentation pages say going with the latest version should be a good move, yet pick which one works best on your iMac, this may require trial and error.
Follow the standard How to create a bootable USB stick on Ubuntu using the Ubuntu ISO you downloaded above.
Boot the iMac with the USB stick inserted, if the iMac does not boot into the Ubuntu installer you may need to reboot and hold down the "option" key, and then selcted the USB stick to continue booting into the Ubuntu installer.