Bug Found in Facebook Sharing

Hello
I have tried to share a picture from the gallery to facebook through your button on the image, link is shared, but the image on facebook is always blank, only a link back, and when i click on it; it leads me to a page where there is like and share buttons at top, and a broken link image.
When checking further, it seems the image is sent to facebook in a different file extension:
My original image uploaded on my website is a PNG, the link sent to facebook is JPG.
Tried to fix myself, but no success with your plugin code
How to fix this please?
thank you

https://ww.wp.xz.cn/plugins/contest-gallery/