Is there an easier keyboard shortcut for taking a screenshot on OSX 10.5?
You can set up a custom shortcut, for example a key, for that purpose.

Open System Prefrences, go to Keyboard -> Screen Shots (or navigate to the Screen Shot-options), and double click on the shortcuts you want to change to change them to the keys you use less frequently. If you have a huge keyboard like me, you can for example set the F5 and F6 to be the shortcuts.

Here is a screen shot of the settings window where you set the shortcut (captured with those as well!) The "Copy"-screenshots are the ones with Control+Shift.