Gotta keep mine in check so people won't find out that I'm a continental buster with massively hypersonic combat speed and reaction.

What is a powerlevel wallpaper?

Powerlevel is, above all, SUBTLE. It attempts to obscure its source material to anyone unfamiliar with it. It looks good to the uninitiated, while providing a neat nod for anyone who knows the reference.

Good examples of powerlevel include:
Insignias and logos
Distinctive equipment or ornamentation (weapons, masks, ribbons, hats, necklaces, glasses, anything that can identify the character to someone who knows the source)
In-universe organizations
Mascots and some non-humanoid side characters. Be careful on this one, it can be borderline.

What powerlevel usually does NOT include:
The anime's name or a character's name. There is some wiggle room here if all other criteria are met.
Faces, or most of a character's body.
Minimalism, if it depicts a character.
Landscapes and cityscapes. There is almost no wiggle room, unless you are 100% sure it's powerlevel despite being a landscape. We know, Monogatari and 5cm/s backgrounds look pretty good, but this isn't the thread for them.
Screencaps, unless the screencaps have been doctored to be high quality.