I think it would be virtually impossible to determine brass from bronze via photograph. eBay is a counterfeiters dream. Even with a piece in hand, I think you would have to resort to chemistry to have a definitive answer.
I've kind of been thinking along those lines. A few pointers I give to folks who ask, is to consider whether the object at hand is typically made of one alloy or the other, for a reason. Fine statuary is made of bronze to pick up fine detail since art bronze expands slightly in the mold before it sets. Ordinary household wallplates for light switches would normally be made of brass because bronze isn't required for that function.
Gunmetal bronze is very hard stuff, much harder than brass. I tried to stamp some small letters in it one time and had to strike the metal stamp over and over.
If you ever really, really have to know and there's money at stake, take the item to a scrap dealer. These days they are all equipped with a "gun" that is a handheld spectral analyzer. it zaps the sample with an intense spark that vaporizes a tiny patch of metal then instantly decodes the spectrum of light emitted, and gives a printout of the top 20 elements and their percentages. The spot chosen to zap has to be completely cleaned of all oxide etc. or all the components of that will contaminate the results. If you know the scrap dealer he might do it as a favor, otherwise they may want to charge you something, or they may not want to do it at all unless you are doing other business with them.
Not all of the guns will ID all of the elements. I wanted to buy some zinc recently and a dealer told me he had no way to tell it from lead or whatever since his zapper was not programmed to identify zinc. That seems strange since it will identify brass vs. bronze, so I suspect he was giving me a line of b.s.