An onscreen ruler is a great tool instead of using an image editor that requires you to first capture the image. Googe 'onscreen ruler' for some candidates, most of them free. The one I've used is at
http://john.kummailil.googlepages.com/downloads.
Here is (hopefully) an image of an onscreen ruler in action:
It is hosted on MSN Spaces, and some filters block it. Be quicker for you to download a free one and see what it does that try try unscrew an image hosting problems, but I'll be happy to email it to you if you want.
If anyone has some tips on using MSN Spaces for image hosting, I would be very greatful!!!
You can also click on the URL below.