My Feedback:
I definitely see a concept working, but maybe make the three postcards more consistent- if you are aligning the quotes to the center, make sure it is done in all three of them (the second desktop version and all of the mobile has left aligned quotes. I think it's okay if the mobile is different than the desktop version in general, but i would keep the two overall groups consistent for this matter). If you want to left align them, make sure you hang your quotes (so the 'W' in 'When' is aligned with the 'w' in 'we' etc. for the other quotes).
Make sure to use a typeface that is not a desktop font, but one that works on the web (look at google fonts, etc.). The serif font works for you, and I like the contrast between the italicized name and the bolder quote, so I would still keep that.
Your letting is very big- both within the quote as well as between the quote and the name, so I would take that down a lot.
Another consistency thing- you use dashes before the name on the mobile version but not the desktop version, so I would pick one or the other to apply for all of the postcards, no matter what version. If you do decide to use a dash, make sure it is an 'en dash', which is a bit longer than the standard one you type on your keyboard.
I think your second photo does not work as well as your first and third, its pretty small because it's offset and has a large black portion which is unlike the other two versions. Maybe zoom in on the photo to fix this? You don't really see what's going on in this photo, while the other two are intimate close-ups.The photos are also pretty dark, but I think the biggest issue is the second one, and zooming in on the scene might fix that problem.
Your mobile versions look like squares right now. Make sure to design them at a typical mobile size (like 375 x 667 or something).