Nice topic! I've wanted to share some photos of what is and isn't collection/extension for a long time but unfortunately a lot of my best examples have never made it to the computer.
Minihgal is in Belgium right now but her girls are the only example of true collection (by classical definitions) I've seen in a mini. Lord knows my boy never got there!
I have lots of great examples of what ISN'T collection from him and I. We were getting good lengthenings though and a very nice working frame at all gaits.
The following are critiques of my own photos for educational purposes. Feel free to ask any questions, I'm posting them with the intention of having others pick them apart for discussion.
This is Kody in 2007 at a breed show. He's working in a nice frame and tracking up but I would by no means consider this "extended" despite his good reach as it lacks power and oomph. You can see that the "V" of his forelegs is wider than that of his hind legs, indicating he's still pulling himself along with his forehand to some extent. This was a nice transitional frame as he first began to elevate his forehand and get his hindquarters underneath himself.
A nice lengthening but he's about to fall on his forehand if I don't rebalance him with my hands.
A few minutes later, still not extended (note that his foreleg is not at maximum extension) but showing much more power from the rear.
This is Kody in 2008 doing a good powerful working trot which could serve as a foundation for an extension. See how he's now moving uphill with his hindquarters deeply flexed and his hind legs working up under him?
A moment of mild natural collection, showing increased bounce and elevation in the stride without changing tempo. His head and neck are not working down to the bit but he's responding to my shortening aids by stepping up under himself with power. This is a good beginning!
My favorite photo, and the only one I have that shows true extension. He isn't overstriding due to his conformation but look at the power from the rear, the uphill slope from croup to poll and the reach in the foreleg! He's giving me the most he's capable of in this wonderful uphill frame.
To be continued....