That’s an interesting point. Again, I think it’s highly dependent on taxon, and what is going to result in the fewest number of wrong selections from suggestions.
To your point, if someone were to post an observation of a dog’s pawprint, and the suggestions all had various carnivoran footprints (dogs, bobcats, foxes, coyotes, cougars, wolves, etc) as the default pic, it’s very likely they’d pick the wrong one. In that case, the charismatic pic helps reduce wrong selections. Ditto for a leafminer’s eating pattern, if it isn’t distinctive…seeing a bunch of moths, flies, etc in that thumbnail at least gives them pause to know that little winding path could be caused by several things.
In OP’s case, the midge may be the more charismatic photo, but it’s far less likely to be observed than the gall. The CV’s still going to be looking at leafy crowns (because that’s the majority of observations submitted) regardless of which thumnail is in place. OP wonders if putting the insect as the default photo may reduce incorrect gall selections on plant observations.
I see OP’s point, but it may also increase the amount of mis-IDed actual gall observations, because someone submitting a photo of an what they think is just a very leafy goldenrod may not select the suggestion with an insect pic in the thumbnail (“it looks nothing like my photo!”)