Screenshots taken with tool "Greenshot" cannot be pasted into editor

Screenshots that are taken with the popular screenshot tool "Greenshot" (http://getgreenshot.org) cannot be copy&pasted into the Confluence editor. When such an image is pasted, nothing happens. But the same images can be pasted without problems into other programs like IrfanView, Outlook 2007, Word 2007 or MS Paint.

Steps to reproduce:

1. Install and start Greenshot
2. Create a new, empty Confluence page
3. Press Print key to take a screenshot.
4. The Greenshot image editor appears that contains the screenshot.
5. Copy the screenshot to clipboard with File | Copy Image to Clipboard.
6. Switch back to Confluence and press Ctrl+V to paste the image. → Nothing happens!
7. Verify that the image was indeed in the clipboard by pasting it into MS Paint or any other program.