I very much agree that we need a more in-depth guide to how to interpret imagery. I'm hoping this will be part of the curriculum development efforts, but as we've seen in some of the posts to this list, there are many issues at play. Maybe having a page at
http://publiclab.org/wiki/infragram-image-interpretation would be good... and I'm sure there's been lots of bits and pieces and examples of interpretation across the site, but to collate and list them, and to derive a more general set of principles, would be great.
I think the most important part will be helping people formulate experiments -- what are you hoping to detect? What is your point of comparison (health vs. what baseline)? Basic experimental design, use of positive and negative controls where it's possible, etc. In your case, do you have a hypothesis about which areas of grass might be healthier and why?
Jeff