I've updated to the last version of magento. Then I changed my watermark that was transparent. Now I see a black background instead of the transparency. I tryed to find some solution, like to create another Gd2.php, but it doesn't work... What can I try?
Please see my answer here (same as comment link just a different answer) - https://magento.stackexchange.com/a/177402/54922
My product png image upload was giving me the same black background. SUPEE-9767 (and the newest magento) extends the validate function in the image upload.
As I do not have enough reputation to comment, thats why I am adding this as an answer.