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I think a common fly should be understood as a commonly posted fly. The chance that the beginner will take a pic of an extremely rare fly resembling a very common one is very marginal.

There are quite a few species that come up all the time:

Calliphora vicina, Calliphora vomitoria, Lucilia sp, Musca autumnalis, Musca domestica, Phaonia tugiorum, Suillia sp, Minettia longipennis, Scathophaga stercoraria, Eristalis tenax, Sphaerophoria scripta, Helophilus pendulus, etc etc.

The gallery should also contain typical flies without precise identification, like a typical Asilinae, or a typical Chironomid midge etc.

As more and more people will take photos, and the gallery here and at other places will grow, the beginners will have harder and harder time to find what they are looking for.

I am not claiming that this would help species level identifications, but would give the beginners some grip as where to start.