How can i have a plain-black wallpaper for Home screen?
make a black image in Paint or whatever. I don't think size matters because your phone will scale it to fit, but it will have to be larger then one pixel. As far as I know the only difference would be how much room it takes on your phone - and a solid black image won't take a lot of memory. A bright-colored, highly detailed photo will take more room then just a solid black image.
And if you want a true black, make it yourself (as you already attempted) rather then trying to take a photo of black - you'll wind up with a dark gray image instead (believe me I tried on my Galaxy S4).