My Wallpaper is Misaligned
[Update September 2021, with Niagara Launcher 1.4+]: If your wallpaper is not centered even though it should be, try this: Open Niagara's settings > Look > and toggle Offset wallpaper.
If your home screen wallpaper gets cropped, misaligned, or not scaled right, please try to apply the Wallpaper with Google Wallpapers.
What causes the bug?
The bug can happen with wallpapers wider than the phone's screen. The problem with wide wallpapers is that either the launcher or the wallpaper provider app has to do the centering (if both do it, the wallpaper will also be off-centered). Niagara Launcher could do the centering, but most Android launchers such as Google's Launcher3, Pixel Launcher, and Nova Launcher don't center the wallpaper; they left-align it (to simplify the explanation, we disregard parallax scrolling). Because of this, most wallpaper apps such as Google Photos and Google Wallpapers crop the wallpaper in such a way that it looks centered for you even though it's technically left-aligned. If we'd now begin to center the wallpaper (like Samsung's OneUI Home Launcher does, for example), apps that wallpapers that get "center-cropped" would now be off-center to the right. Because most launchers left-align it and, therefore, most wallpaper apps crop them, we decided to also left-align them.
My Wallpaper is Cut Off/Doesn't Fill The Screen in Landscape
On some devices (specifically from Xiaomi), the wallpaper is cropped when rotating the device to landscape, leaving a black bar on the right side of the screen. In this case, we suggest applying the wallpaper image as a live wallpaper instead, for example with KLWP, Muzei, or Wallpaper Changer.