I did some testing on my Mac and if I drag and drop, but don't add any message, it shows up in my webmail as a PNG that is not shown in the message viewer.
If I add text to the email, it shows up correctly as a PNG and views in the message viewer.
If I add text to the email and use the attach button, it shows the image in the message viewer.
In all cases nothing changes about the file. It is still a PNG and it isn't altered.
All received messages indicated image/png and inline disposition.
Their email client is clobbering the attachment, not Mail.
And, just to confirm, you are dragging and dropping a file, not an image that is open in some other source such as Web Browser or Preview?