I think that's the artist you're looking for: beanbean1988.

Generally I would advise against big images as they make your content look smaller/less than it really is. Additionally, they can be distracting. In most cases you see banners like this

in Mobafire (Image from "Internet Explorer Ezreal" Guide).

I don't think Lux is really underrated xD
Nevertheless, I will give you some advice. Be careful of using too huge images and too many colours. It can look pretty crowded and messy.
Furthermore, add some tricks, for example using E to slow the opponent to have an easier time to land Q. The details distinguish the good guides from the great guides.

You can put a to-do list into your change log if you have still content coming :)

Looking good! Love the "UNLEASH YOUR RAINBOW POWERS" as you put it, lmao, one thing i found interesting is that you put deathfire touch instead of thunderlord's which is just so dang strong right now, and actually goes quite well with lux, i recommend you try this out, im not much a lux player but i would like to learn her, but yah keep it up! would love to see this guide become the next best lux guide!