Hi,
There is some Freeware called "FastStone Image Viewer" that is idea for cropping exactly. Its crop tool opens the picture again and "greyed". The size of the crop can be entered and then appears in normal colours and can be moved around.
It's very much like using an enlarger and paper and chemicals, as the crop can be seen and moved to suit.
For 9 equal segments you'd just need to divide the size of each side by 3 and add a bit for overlap - sorry if this states the obvious but I have been posting here for a long time and think everything has to be spelled out in full.
I often chop pictures up into halves or quarters, with an overlap and then print at maximum size and glue together. You can print on paper, turn over and put stciky tape on the edge then peel off to thin the paper, on both sides of the join. Use "Postit notes sticky side upwards to get the join right and then turn over carefully and glue with long term sticky tape. (This is where I remember "Butterfly" sticky tape and sigh... )
For hiding the joins, look where the picture will hang and arrange the overlap so the shadows of the normal light falling on the picture will hide the clean bright edge of the individual sheets. Usually not noticed, especially if hung where furniture makes it hard to get near the picture (about a foot is enough).
I keep meaning to experiment with overlaps like brick work: so that the join doesn't run the full height of the picture and (say) the bottom is two halves and the top is a quarter, a half and a quarter again. But I never have the time.
BTW, it's a usefull trick to find out if a picture can be printed at (say) A3 as the important bit can be included in a half crop and then printed on A4 to see if it works. Cheaper still to crop out a quarter and print on A5, BTW.
Regards, David