I need make a restrict area, and there have somes images.

I need a tool that i can measure a piece theses images, 

and i can see the result in pixels, cm and mm.

For Example:

There is a image with 1000x1000 pixels.

The user open this image in his pc and select a tool.

WIth this tool, the user select a piece of image and can see

the size of th piece selected.

One Suggestion?

Thanks...

Jardel