Are you sure about that? How does 300 ppi not look better than 72? Wouldn't a higher resolution image that you downscale look better than an imagine which has a low resolution which you blow up?
The size at which an image appears on your screen depends only on two things - the pixel dimensions of the image and the display resolution of your screen. As long as you've set your screen to its native display resolution, then an image will be displayed pixel-for-pixel. In other words, each pixel in the image will take up exactly one pixel on your screen. For example, a 640 x 480 pixel image would fill a 640 x 480 pixel area of your screen. An 800 pixel-wide banner on a website would appear 800 pixels wide on the screen. No more, no less. And no matter what you set the image's resolution to in Photoshop, whether it's 72 ppi, 300 ppi or 3000 ppi, it will have no effect at all on how large or small the image appears on the screen.
That's because image resolution affects only one thing - the size of the image when it's printed. By setting the resolution in Photoshop, we tell the printer, not the screen, how many of the pixels in the image to squeeze into an inch of paper. The more pixels you're squeezing into every inch of paper, the smaller the image will appear when printed. And generally speaking, the more pixels you're printing per inch, the higher the print quality.
Oh, well your second paragraph makes more sense. I assume the increased resolution for parallax is simply there to compensate the fact that using parallax will lead the iPhone to zoom in a little, hence you'd need a higher resolution to compensate for it? Not to mention the that it also "moves" around.
Yes.
So if I'd like to contribute with an image and ask you to edit or downscale it, what would be the required resolution to get without sacrificing image quality? I do find wallpapers set for 1334*750, but using those for parallax will make you lose detail as it isn't the exact resolution when using parallax, right?
The perfect ratio would be 10.5:16. So anything 872 x 1635 which is a ratio of 10.5:16 and above. Sometimes below this is possible due to the fact that there are programs available that allow for resizing up. But they do have there limits as to how much upscale they can do before you start to lose quality in the image. Sometimes being slightly off of the 10.5:16 ratio can be worked with as well. But there will always some distortion that will happen if the full image is to be fit within that 10.5:16 ratio, unless some of the image is cropped away. With geometric shapes it is hard to notice sometimes, whereas images of a subject will look strange.
You'll see that they offer a wide variety of resolutions, even for the latest iPhones. But, they only offer the non-parallax resolution. So how much more would you need in order to edit it into 872*1635?
It would not be hard to size an image at 750 x 1334 up to 872 x 1635. That is a minor upscale. The program I use for scaling images is BlowUp 3 by AlienSkin Software. onOne Software make a program for this as well called Perfect Resize.
I hope this helps.