One issue we've noticed so far is a large number of users saving edits from outside the "add an image" workflow.
That sounds like exactly the success we want, assuming they are having even marginal success. If any of these users are finding and adding a different image, and that image is plausibly equal or better than the one you suggested, that is a huge success. The proper way to do this work, really, is to look through the relevant available images and use good judgement to select the best one.
If a user stays in Add-a-link just making lots of links, there really isn't much value in that. What we want are tasks that lead people to the general-purpose editor. Our lifeblood is an on-ramp for users to progress into the full range of work we do. We want them continuing to gain general editing experience and knowledge, to gain the expertise for more sophisticated levels of labor, to be able to oversee the next round of new users. Only a few will stick around to become high value expert contributors, and we need some rare individuals who promote to administrator.
Your goal is for some of these people to eventually make the circle back here, offering years of insight and field experience to some team working some future project.
You may need to carefully reconsider metrics for these kinds of tasks. It's not really a win if someone is parked in the structured workflow grinding the basic metric-count higher and higher. The big win is when they discard the training wheels and keep editing.