How to adj wallpaper re: less grainy

[Glennster]

I currently have an iPhone 8 & found the attached custom pic online. However, when I use it as my wallpaper it looks grainy & not crisp.

What causes this?

How can I adjust this so it looks more hd quality?

 

[tehStickMan]

It's a small resolution file, expanded into a bigger resolution (which is still too small). Find a better one.