I think you've established a pretty identifiable tone/concept with the combination of black and white images with an elegent, light weight sans serif font. I think even though you've established a contemplative tone, you still might need to push the visual interest of the quotes to engage the viewer. Consider whether you can do something stronger with the quotation marks, or maybe makign some kind of mark because the simplicity of the images combined with the simple approach to the text is making it feel a little static. I would also try to unify your choices regarding color because it's a little uneven currently. I might also try a weight shift between author and quote, but the uppercaps are working really nicely. Maybe think about having some sort of logical progression through the images; maybe he starts as a young man and gets older - just have some sort of identifiable flow through it so it's not just like "here are 3 photos of john cage."
I think this is the strongest of your three. There's nice seperation between the text and image so that it's easy to read and understand both, and there's clear space for the text to flow into when the screen size changes. It bothers me a little that the space relationship between the quote and his name changes between the two sizes - if you decide to make that change I would make it look like an intentional choice.
This one pulls away from the other two for two reasons. One is the use of color in the background. I think it looks pretty compelling and works aesthetically with the black and white image, but it does make this postcard feel seperate from the others. I would consider using this blue in the other images or going to all black and white. The second reason it pulls away is that the photo seems really different from the other two images of john cage (?). It looks like someone performing an action, but I can't tell who it is (sort of looks like reagan?) and I can't tell what exactly they're doing and what their relationship is with felix the cat. I might just pick a different image. The letting is really huge - consider the typographic choices like you would a type project.
The text is definitely getting confused with the background here. This could be solved by making a global choice to use a heavier weight text, or using photoshop to darken/solidify that background space a little bit. I think that would be the better choice because it would allow you to keep that quiet tone. I also think the composition on the larger format is a little heavy on the right - maybe try photoshopping to make the right of the image extend so you have more negative space to work with.