You are pre-defining your tileset, don't do that... And to topple some more errors on it, you added the +name, this will end up like this: "Spring_PkmnBlackWhiteSet.pngPkmnBlackWhiteSet.png" this is why it doesn't work.
you should have left the script completely clean and just added the images, this would have worked.
Your script won't work:
you are adding the .png where it is not needed...
Anyways, to fix the problem do this:
1. Insert a CLEAN UNMODIFIED VERSION OF THE SEASONS SCRIPT
2. Don't modify it
3. Make sure you have proper image tilesets for your seasons (Summer_blah, ...)
4. Test and report bugs. (if any)
However, my script doesn't include an unnecessary .png in it. That's not to say it doesn't work, but I'm just letting you know that my script has absolutely no unnecessary .png file extension references.
But if it doesn't work, it doesn't work. I didn't intend for it to be a solve-all problem fixer. It's just an old script I had on hand that I modified some time ago.