How do I install Linux on an Intel iMac with no OS nor CD drive
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.