Hello again! Now, I was wondering this one basic thing I just attempted doing out of boredom so, let's say I found a cool image on some social media I'm in, and that I wanted to make a wallpaper/background for myself with it, and I've been working on it for few minutes; once I'm done, I try to set it as my background, but guess what: it looks kinda pixelated and like if it was in low quality, while I look at it in paint.net, it looks completely fine. What can I do to get it to work properly and look "full HD/quality" for my background? I already tried resizing and using templates, but it won't work.
Anyways, here's the picture, I didn't add too much detail or effect into it, or at least I wouldn't consider it anything at all; I'm not done with it, and I still need to get rid of the white dots around the character and other stuff alike, but I'd rather get this doubt out of my head before I work too much on it...yeah.
It looks fine in the Windows visor, paint.net, and everywhere else, but as my screensaver the quality goes lower or it gets pixelated, what can I do to fix it, or to make it higher quality as a background/wallpaper? Thanks, and good luck! PD; The character, and the drawing aren't of my creation, I haven't asked the creator for permission neither, but I was just using it to have something to use within my daily dose of randomly improvising with pictures, if asked, I'll take it down.