I'm afraid there's no ready to use solution.
It's easy to make the whole window transparent (or adjust the transparency), but using a transparent image as window's form might be harder. Especially for LCL, due to LCLCarbon internal implementation.
If you could find a C-sample on how to do it, using pure Carbon API, I can help you adopting it to Lazarus.
p.s. But i'll be away until 29th of december.