I was thinking this exact same thing the other day when I re-read that part of OoTP. I'd never noticed before that it didn't explicitly say Fabian was there too. I think an author can have a free rein with this. It can be inturpreted that by the absence of Fabian's name that he wasn't in the Order, or it could just be that Fabian wasn't standing near his brother in the photo and Moody just didn't want to find him and point him out to Harry because he wanted to show him his parents. I think it could go either way.
Is Fabian Prewett in the photo? Here are Moody's exact words: "Gideon Prewett, it took five Death Eaters to kill him and his brother Fabian, they fought like heroes..." It seems to me that he would have said something like "Gideon and Fabian Prewett, it took five death eaters to kill them, they fought like heroes" if Fabian were actually in the picture. Or maybe he would have pointed Fabian out as he said his name? I'm not sure.