Where can I find screencapture to replace mine that is broken? Some days ago I've accidentally replaced my /usr/sbin/screencapture with a newer one from 10.8 with the hope of getting more options for that command.

However now it doesn't work and I'd like to get the older one (compatible with osx 10.5.8) to fix the problem. 

Where can I find it?
If you had 10.7 or a newer version of OS X, you could download an OS X installer from the App Store and use Pacifist to extract the screencapture binary from the installer. I'm not sure if something similar is possible with a 10.5 installation DVD.

Alternatively, if you upgrade to a newer version of OS X, it should also replace the screencapture binary with a new version.