1. For the third one, make sure the text is all in the same weight.
2. Perhaps work on the rag a bit more for the iphone screen in your second one
3. In the first one for the mobile, push the image to the left slightly to center the two characters and get the awkward foot out of the scene.
Extra note: Is the photo the same on each version of the computer and the phone background? I think I see some added black kinda wash over some versions but not the other. If that's true maybe try to avoid having to change the actual content of the photo based on the size? Like resizing is good/cropping diff but I'm not sure if adding anything is right? But maybe that is allowed, I'm not quite sure if thats OK or not.
Overall, really great job. I like that you added a dark black background for the background of all your text to keep from any type of distraction. You have a good overall concept and aesthetic and the photos all match the quotes nicely. Also make sure it's a google font! (I can't tell if it is or not!)