Setting a wallpaper without stretching it

[AC Question]

Hi,
I was trying to set a new wallpaper for my lockscreen but for a reason I don't know, it keeps stretching even though I do like always. I'm using the image app that comes by default with the phone. I have a Samsung Galaxy S5 running Android 5.0. Tried Wallpaper Wizardrii but it does the same thing. The only thing that "works" is by going in the setting and changing it there but it seems like the image quality is lower. Anyone have any idea what's happening ?

Thanks !