How to protect images at web by convert it to images.php?

Hi,
I have some problem facing for my copyright of images ay my website keep illegal downloading from people. My images I have put watermark, but they still keep downloading.
I have listen before have programmer they convert the images to extension images.php instead of images.png. So I would like to know have somebody here know this way of protecting? I really hope can get some suggestion here and solve this problem. I know that I can't stop people from printscreen and get my images but my images basically is .png form that means is transparent background, so if he printscreen that means he need to cut again the images. So the better way they do is just downloading but not printscreen.
Thanks and wish you all a best day,
Connie
