Finder/Quick Look Malfunction after 10.8 OS upgrade

As already mentioned ImageMagick is ideal. The other thing is to use Automator.app to convert. It comes with the Mac and some people find it less intimidating than the command line, presumably the point of having it built-in.

Create a new "Application" in Automator and add the "Change Type of Images" workflow action. Set it to convert to JPEG, then save the application somewhere.

When you drag an image onto the application, it will convert it to a JPEG for you. If you don't want to delete the original photo, then add the "Copy Finder Items" action to the top of your workflow.

It is not possible in Mac OS X itself, since Space does not actually activate a menu item, and the title of the menu item in the File menu changes with the name of the selected object(s). You could learn to press Option-Space for fullscreen mode.

Alternatively, you can use hotkey/utility applications such as Butler or Keyboard Maestro for this, to intercept your key press and reroute it. The following steps are for Butler.

1. Add a Smart Item » Keystrokes to Butler, and move it to the Hidden category.
2. In the Keys tab, press Cmd-Opt-Y (which is the regular Finder key combination for fullscreen/"slideshow" Quick Look.
3. In the Triggers tab, press Space as hotkey, and confirm the dialog questioning your sanity.
4. As Exceptions, select Only, and enter "Finder" in the text box.
5. Enjoy!

A significant downside to this sledge-hammer-solution: You cannot ever press space in Finder to have it mean something else. Renaming folders, searching in Spotlight: Space is not going to work.