After installing Yosemite, opening iPhoto brings my iMac to a halt
So everything was working fine, you opened iPhoto, and wham! ... a non-booting Mac? Something else has gone wrong here, and I doubt it's iPhoto. After installing Yosemite, did you do other things with it successfully, like browse the web, open a Word document?

From the screen you describe, the HD is the problem. Either it is completely dead, system files are missing/corrupted, a loose cable ... anything along those lines.

If you have a Time Machine backup of your HD, use OS X Recovery to recreate your startup disk.