Are *all* of my photos in iCloud?

What it is enabled is Photo Stream, a photo sync service among all your iPhones, iPads, iPods touch, Macs and PCs. All photos saved in Photo Stream are deleted after 30 days, so none of your photos are stored in iCloud yet. Apple is testing iCloud Photo Library, which will allow you to save your photos in iCloud, and it will be launched soon in OS X.

Thank you mende1! So, I have to use a 3rd-party backup service if I want to ensure I don't lose any of those photos, correct? Does anyone have a service they really like?

If you have Time Machine enabled, and if your iPhoto Library is in the default location in the Pictures folder and not on an external drive that is excluded from your backup, your iPhoto Library will be backed up by Time Machine. Or simply make copies of your iPhoto Library on an external drive or use cloning software that can make incremental backups like Super Duper or Carbon Copy Cloner. You don't need a cloud backup to keep your photos save. Any copy on a different drive will do. Indeed, having your backup on an external disk that you can readily access, even if the internet does not work, is faster and more convenient, than to have to download thousands of photos again over the network.

My main concern is fire and/or theft.

I keep an additional backup drive in my office at my place of work. You could also simply store an additional backup drive outside your home or in your car. So, if your house should burn down or the a burglar steels your mac, you will still have the backup drive outside the house.

But iCloud Photo Library, that mende1 mentioned, will be available soon. It already works for the mobile devices with iOS 8, and is currently beta tested for Photos.app on Yosemite. You will soon be able to move your whole iPhoto Library to iCloud, but it will be a paid service, if the 5GB free iCloud storage do not suffice for your library.

See this link: iCloud Photo Library beta FAQ

And also: http://www.apple.com/icloud/photos/