I've been using F-spot to manage photos for ages, and my Ubuntu box defaults to using it even with the latest releases (with Unity). However, I just created accounts for the children on the machine, and wanted those accounts to use Shotwell instead. So, I have both installed, but one account must use F-spot and the others use Shotwell. Could I find the instructions on how to do that? Nope. Well, yes, but only after a lot of looking.
There is a preferred applications options in system settings, but this doesn't cover the photo manager - which it should.
In the end I found an option in Nautilus (that's the Ubuntu/Gnome File Manager), so if you go to nautilus/Edit/Preferences/Media tab, you can find options to set various default applications. The one you want is Photos!