Really need help finding an application for help with a PhD project

[MasterHowl]

To cut a long story short...

I'm helping out with a PhD project and we're collecting data from planetary satellite images on the sizes of impact craters, and the frequency of different sizes.

This is done by looking at a crater, selecting any three points on the crater rim, taking each of the point's (x,y) co-ordinate (using the pixels), and from that you can compute the size of the crater.

Currently, we use our "favourite graphic software" (I use Pixelmator, others use CS5, others use MS paint...), since they all have coordinate systems. We have to manually record the three coordinates.

We need a piece of software where we can literally just click three points, hit enter, and the coordinates of the three points will be outputted to a text file, where each line in the text file has three coordinates (one for each point).

Is this possible or will we have to write our own program in Linux or something? I know it's a long shot, but thought I'd ask here...

[blackboxideas]

I'm not sure what size images you are dealing with but this could help?

http://bigwww.epfl.ch/thevenaz/pointpicker/

Edit: just had a quick search MapAnalyst could be what you're after!
http://mapanalyst.org/man/points.html