I use the feature ‘Import images to group Assets’ in the Markdown Files preference.
I am copying a black image in png format and “paste” this image in the Assets folder manually. This will create the image in the folder and when I select it it shows the image in the original colors. All good.
If I now go to a markdown file and past the image in the document it will create a markdown reference to the image and also create the image in the Assets folder. However in the preview of Markdown the image has a gray shine on top of it. Also when you select the image in the Assets folder that same grey shine is visible. So importing an image does not work properly.
If I now copy an image in JPG format and past in into Assets I see that the image is created as a JPG and the colors are good. If I then past the JPG image in the markdown image I notice that the image is created in the Assets folder but also converted to PNG. The grey shine is not visible.
So summarizing copying as PNG does not work properly and copying as JPG does work. See the screen shots for the difference.