PNG images with a transparent background are not transparent.
Look at the image emacs_png_bug.png on the attachment.
This is my first bug report. If I am missing something please let me know.
Below are the commands i used to compile the emacs.

git clone https://github.com/emacs-mirror/emacs.git
*cd emacs*
*./autogen.sh*
*./configure*

*./configure --with-mailutils && makemake install*