I'm not sure if You have tried this, I'm not 100% sure, that it will work. I had a similar issue, and I've been really pissed off, because I was sure i do everything just like in the tutorial, so eventually I created the most basic tutorial example following it step by step, even with the names. It worked. I tried to find the difference between my mod and this simple generic tutorial, but I couldn't find any.
Then I messed up a little with the basic example to see how I can make it screwed again, and i turned out that UPPERCASE LETTERS WERE THE CASE. I renamed my mod folders to all lowercase, without spaces, changed modID to lowercase and it worked.
Png files themselves didn't have to be changed to lowercase, thanks God. Shitty bug if You ask me.