I have added textures with kgpanels succesfully, so I tried a similar path for the texture, but I am unsure if the problem is with the path or with the texture itself.
What size should it have? TGA I assume, as the other Addons are using?
I would prefer a recolour of the whole Icon or a fat frame around the Icon instead of a thin ring or stars.
I had Hekili running too, deactivated it, reloaded UI and MaxDps did not seem to work anymore.
It started working again after a reset.
Note: I would like to see the Addon marking the next Ability while it is on CD, so you can prepare better for what is next.
As for adding textures u need to follow the common rules for addons. There are a lot of info about this on google.
For example this video: https://www.youtube.com/watch?v=93E_bXikHA0