I know the photo you're talking about. Sometimes photos like that come from the Wisconsin Historical Society's* Wisconsin Historical Images collection, but that one doesn't appear online (though what's online represents only a fraction of the photo collection). If you didn't see a photo credit in, say, the corner of that particular mural-sized photograph, I'd frankly contact the Brewers and ask where they got it. I'd be surprised if they weren't willing to tell you the source, though if it's their own records they may not be willing to share a copy. We have club seats for a game next week, so now that I'm curious too, I'll try to remember to have a peek. (*Disclosure: WHS is my employer, though I don't work directly with the image collections. But if you are interested in a tease, my current avatar is from the collection linked above. Do a subject search for "braves" and enjoy the results.)