Please note that those artpacks are exclusively for use in Battlegrounds: RPG Edition and/or Battlegrounds Gaming Engine (both are VTT programs).
The media they contain cannot be extracted for use in other VTT apps.
The artpacks are provided for free to help offset the cost of the software itself, and as a "thank you" to BRPG and BGE users.
Roll20 premium edition does just that after setting up fog lines.
This also means you can set up light eminating from a player such a torch, and with the API/DEV tools there are some nifty codes to have torches slowly burn out over time. Edit: Albeit these extra codes are highly experimental and will require some reading for your to implement.
You can also look into virtual tabletops that are out now such as Fantasy Grounds ($$) or Maptools (~~$$~~). These will provide grid support with layers for tokens, fog, and the map itself in a very similar fashion to what you are doing. You simply click and drag (or outline) and remove any fog masks to show what the character may see.
Finally another option is Battlegrounds although I am not familiar in how it works or the current development support.
Edit2: The existing version of d20Pro (now called d20Pro Legacy) is available today at www.d20pro.com. I'd almost stick to what you are doing now. It's incredibly similar to what most of the tabletops do, and you've developed a method to do it for free without having to worry too much about matching gridlines.
Edit: And you can always keep an eye on d20Pro although as before- I'd be wary of backing more projects and simply keep up to date on how the development is going and maybe snag it at release.