This is as always just the start of it all.
There’s a bunch of different apps that got wallpapers, some a lot of various (a lot rubbish ones). And others got less but more quality.
Of course. good old Uncle Google got you covered in this as well. Just do am image search for whatever topic you want, and under “search tools” you find “size” where you select “large” or use “Exactly” where you enter the correct size for the display you’ve got. Like 1080 x 1920 pixels for One M8/M7. Or just google for “wallpapers for One M8”, etc.
Take a photo yourself
Gets you the best wallpapers.
Use a computer
Use Photoshop etc to either make a whole new wallpaper or to edit a photo.