I hope this is what you're looking for. I don't have that much experience with making wallpapers, but I tried with Sai, and went with something kind of simple .. As far as BG goes and, I guess, models .. >.<
I got the chibi neko Rin and Len models from this video in the description ::
KAITO came with the mmd program. The background I made in Sai. I positioned the models. There are many ways to interpret this, but I was going for something like Kaito being the father figure of the two, but you know, not actually being their father.