There were quite a few sets of instructions that were completely subjective, but a good number of them were pretty accurate and had chosen strategies that, even if they didn't specifically mention pixels did a good job at representing colour, position, and scale. Notice that a couple of students included colour into their instructions, although only one of them made sure that the colour they drew was going to be the same one as the source image by including RGB codes.
My next lesson with these two classes will be students trying to recreate images, establish what algorithms are important, and then work on an algorithm with the class to follow when writing and reading image data for simple images. Depending on time/progress we'll look at colour, particularly since Piskel maintains a numbered swatch panel, which might initially look like an attractive way of recording colours (à la indexed colours).
My two classes of year 7s really got into this, and handled the discussion part of the task quite well. It was a more difficult task making sure that the pictures they drew in Piskel weren't too complicated (I had houses, cats, a Nintendo DS, all multicoloured, before I reminded them they had to write instructions on how to draw them!). The adversarial side of the task was also a big incentive and the kids responded well to that part.
The difficulties mostly lay in the technical details:
Since OneNote Class Notebooks have been made a bit easier to use with (some) SEQTA integration, with my next batch of students next term I will have a go at using the collaborative space to share instructions and the resulting images, although OneNote has been quite flaky when dealing with a number of students and images at once.
As far as the core ideas I wanted to get to went, I think the activity was pretty successful, particularly when we got past the talking stage and into individual problem solving. Next year when I have more time with my year 7s (this year I only see them once a week for a term), I'll work it into a more fully featured task.