I don't know, unfortunately. I tried to do some research on it yesterday, but it was not that easy. Apparently there are also CF_DIBV5, CF_BITMAP and something called a "png" format for clipboard, but I don't know why CF_DIB doesn't work always, and I don't know what other formats would work in e.g. chrome.
All I know is that it would be very useful to be able to paste graphs directly into slack or emails.