In this case, I would suggest simply renaming it from "Last update" to "Last significant update", unless that would entail changing too many dependent nodes.

For completeness I would also suggest adding the fact that a changed image will have the same effect as checking the "significant update" box to the help.

mhi might one day, when he's farther along the road of attaining PM sainthood, humbly write patches like these himself. At this point he hasn't even quite figured out yet which monks pray and fast over which monastic code... :-)