Look into competitors, and check for ranking wallpaper keywords, optimize your images as well. Target google search attributes and again, more competitors research and monitoring like anything else, comes down to optimization on your end. Simple example would be ( Black-hat-world-seo-wallpaper-01.jpg) or something of the sort rather than default, otherwise unrelated potentially, or generic titles.
Keep file sizes smaller to ensure shorter load times on each page, compress images. Of course the more links you get from (high authority) sites, the better your chances of ranking, so it’s in your best interest to have as many people see and link to your content as you can. This is a good concept to bring about interacting and reaching out to other bloggers and sites, so that you can build relationships that have a mutual benefit too, a big part of SEO will always be networking.
Posting daily, multiple times is always ideal, and don't forget filler content, or body text, some competitors might be flawed in comparison to what you can accomplish if there's a lack of said content..
Other factors on the simple side, wallpaper URL should also have the keyword you are trying to target. There's lot's at play we can definitely talk about many things, maybe time to reassess the inner workings a bit. Also making sure google is redirecting to you through hotlink protectiion.