Unity is meant to only include files that are actually being used. Using the Resource folder overrides this behavior. Asking your users to remove the files manually really defeats the purpose.
The shaders don't have to be in there anyway since they will automatically be included if a used material references them.
The textures could as well be linked on the shader file itself:
This can be done with scripts as well, so there is no reason to resolve the textures in "OnRenderImage".
If there's a case where that really won't work, use the Asset Database at edit time to assign the texture to a serialized variable, so Unity will store it and include the texture in a build.
I hope this helps you improve your product. The effects are great, so it is a shame the current resolution is so wasteful.
Thank you for you reply.
It seems it may not be the problem of the shader script nor the Plugin.
Instead, it seems is the problem of the camera setting. I disabled HDR on the camera and it works probably now. (I also updated unity version from 2017 to 2018.1.1f1 in between but I am not sure if that affects this)