There's no doubt that as a state, Montana is overrun with postcard-worthy views. I've always thought, though, that the very best of those scenes were of Lake McDonald, up in Glacier Park. Most postcard photos of Lake McDonald are shot from the Apgar area, looking down the full length of the lake at the impossibly-beautiful mountains that frame the opposite shore. But I like this closer view even better, taken from the boat dock at what is now Lake McDonald Lodge.
This photo is probably from the first decade of the twentieth century, when visits to the lake were becoming more popular but before Glacier Park itself was formally established ... the caption makes no mention of a national park, but simply says "Lake McDonald, Northern Montana."