Aesthetically I think the tone is a good match for the quotes and subject matter.
Aesthetic is consistent between all three images; image treatment is the same.
The very first web postcard is really clear and sharp because of all the white space the text has to live in, but for a lot of the other cards the text becomes lost in the grey/sepia of the images -- it might be worth fading some of the image edges to give more space for the text to live in.
I hesitate to suggest adding weight or color to text, as it would alter the tone, but that could be a solution if you are willing to explore a different tone. The white on that delicate serif is a little difficult to read when the background is alternating mid tones, so changing something about the font to boost the contrast could be a solution.
The priority on each of the cards is to me focused on the text itself rather than the images -- they are prioritizing the message of the quote rather than the content of the images. (I'm not suggesting this to be a bad thing, only what I consider to be the visual hierarchy from what I see.)
I consider the text to be foreground, image to be background.
I think it would be easy enough to program the text to be able to shift or move to a new place as the screen width collapses, I think you should be careful where that text shifts on its own -- its considered in a still image but in an in-between size the white text might have issues over the light spots in the background. Great contrast in the darkest white space, but loses a lot of it as it touches the image.
I think you have considered all the edges, your have images fit at both web and mobile -- The second image where Dumbledore has his arms spread becomes a little busy at the mobile size because of how much goes on in the enviornment behind the figure.
I certainly think this is buildable, the design clear and efficent, I think it just comes down to make the text legible at all places it sits over the image.