Here's a grid that should work for you, although I based it on the size of your image at 100px per square. Kinda presumptuous of me now that I think about it...

Click image for larger version. 

Name:	a-grid.png 
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If you let me know how many squares your image is I can fire up another one. Does .png work to import as a layer into GIMP?

I'm looking forward to seeing how this map develops by the way!

cheers,
Meshon