I found this one at the macrumors forums, so I take no credit it for it, but it's awesome.
Most people I know have reduce motion turned on. I didn't get the big deal with the Parallax Effect. I use regular size wallpapers. I'm still on my iP 5, so that's 640x1136. I have noticed in posting wallpapers for the new phones, that the 6 size of 750x1334 will post using TapaTalk. But the larger Plus of 1080x1920 or higher get reduced to 576x1024 on TapaTalk. A good resize app for wallpapers is "image size". It has a minor bug, but Vitalij Sch?fer, the developer, told me it will be fixed ASAP. He just updated 2 days ago. So you have to run the wallpaper through 2 or 3 times to get it right. But it works great despite the bug. I use it a lot.I've read that the ideal resolution, with parallax, is 2662x2662. Is anybody making wallpapers this size? So far it seems like everything is 1080x1920.
Most people I know have reduce motion turned on. I didn't get the big deal with the Parallax Effect. I use regular size wallpapers. I'm still on my iP 5, so that's 640x1136. I have noticed in posting wallpapers for the new phones, that the 6 size of 750x1334 will post using TapaTalk. But the larger Plus of 1080x1920 or higher get reduced to 576x1024 on TapaTalk. A good resize app for wallpapers is "image size". It has a minor bug, but Vitalij Sch?fer, the developer, told me it will be fixed ASAP. He just updated 2 days ago. So you have to run the wallpaper through 2 or 3 times to get it right. But it works great despite the bug. I use it a lot.
Image size by vsmedia.de
https://appsto.re/us/oxX-N.i
Sent from my ancient but trustworthy iPhone 5
Square? Depends on the wallpaper. That works for the iPad, but don't know about the Plus. In any event, posting any size for the Plus here lowers it to 576x1024. It may be an app thing. I use TapaTalk. Posting on the online site may be different. But there's been issues posting photos on iOS 8 in Safari. Why do you like the Parallax Effect? I didn't see it was that great. May be on a large screen. But on my 5, it wasn't much different. The "effect" was minimal.Personally, I prefer to have parallax on. And based on what I've read, 1080x1920 is not the optimal resolution; it is actually 1242x2208, and the phone downsamples to 1080x1920. Also, since the 6 Plus can be used in portrait or landscape, shouldn't the ideal wallpaper be square?