I have not installed in on my N900, but have it on my desktop. You could probably use

http://maemo.org/packages/view/imagemagick/

and use that to apply "import screenshot.png".

Edit: Coulnd't help myself, even if I wanted...

So, I tried the maemo version, and it works. Keep in mind you need to give it a mousepress after giving the command/enter in order to initiate the screenshot. Also keep an eye on your location and either move into the folder where you want your screenie to be saved, or add it's pathname on the command line.

Easy enough?