How about mentioning OpenCV?

For a closing, here's a first idea:

The choice, then, of image processing package depends on the several factors including speed, correctness & quality, growth, community support, the surrounding ecosystem, algorithmic variety and users' ability to contribute.  Sometimes, benefiting one factor comes at the cost of another, e.g. our approach of writing code in a high-level language may affect performance, or strict review guidelines may hamper algorithmic variety.  We motivate our design decisions for scikit-image in Section x.x.

We should then also mention these aspects of the packages above, if it's not already mentioned (and the list is just my first guess, we may have to expand or reduce).