So you're making an offer that you clearly didn't made and expect Mystery and PK to make it ? Seems legit x)
I actually like the white background.

P-probably...uhhh...sounds shady...white is cool and all but I guess having more backgrounds to choose from would not be such a bad idea.
It's not like it makes that much of a difference.

wut

But anyway, you can change the way the forum looks. Just go to settings and then choose one of the 4 styles available. (not exactly what you want, but eh)

This

Nah, seems like a pointless idea. Select wallpapers? Maybe. Uploading your own though? Definitely no.

Also PK and Mystery have no impact on the forums, that's either mkerix or PewDiePie himself :V

Not to mention people will probably break the PG-13 rule

What the above said. You wouldn't wanna go onto someone's profile and see this gigantic 1920x1080 image of someone's dong.

Size is not the problem, the content is.

And will that rule be followed by all the users? Not very likely.

I don't think this will be added anytime soon but in my opinion, choosing from a set of wallpapers provided by the site would be the best idea.

Aight, first off, not me nor Peace has the full right to bring up ideas to the team. I might be able to propose it to the team, but even then I'll need constructed arguments why we should have this.
First off, this is a forum. Not a social media thing. You could maybe see it as something like that, but it isn't. Backgrounds are all the same, and the only edit you can do is by switching themes.
Secondly, there are some practical problems to be overseen. I know this forum board, or any forum board, has no support to make such changes. Maybe it has, but I'll need to ask Heiko and q23p if there might be a probability it can be done. It'll take a shitload of time, though.
Lastly, the size of the images. A normal background would take up at least a few MB. If everyone, let's say about three thousand users, upload such images, it'll become a few TB.
It's a nice idea, but I fail to see any real potential.

Pointless idea is pointless and only will make loading times way bigger with it. Not to mention your cache and such if you visit a lot of customized profiles and such. Not even talking about potential space on the server if it's hosted on there.

Sexual Photos Everywhere,God Bless who sees one.

This also, think of huge lewd pictures decorating someones profile. Lovely.