How to get a picture from the internet to my iphone wallpaper?

I have done it but recently the dimensions won't change and configure to my screen. If i click save image then go to wallpaper and try from there the picture is to large.

If you have updated to the latest IOS version, in photos there's an edit option. You can edit/crop the picture to the size and shape that you prefer.

1.
Tap the picture.

Make sure the picture controls are displayed.

2.
Tap the icon at the bottom left (the one that looks like an arrow trying to escape from a rectangle).

That displays the four choices shown in the figure.

3.
Tap the Use As Wallpaper button.

Why don't you try free wallpaper for iOS device? They will fit your screen perfectly!
https://itunes.apple.com/...19159?mt=8
Its free for use!
Hope it helps!