In Firefox (in Windows, Linux and OS X), it is possible to drag an image from the desktop into the paste tool and paste it into the editor. This works as well in Debian with Iceweasel, which is based on Mozilla. Simply open the paste tool, drag the image into the tool, and click OK.
With versions of ckgedit and ckgdoku dated June 23 2017 and later, using CTRL-V, or your system's equivalent, it is possible to insert images from the clipboard directly into the editor in most browsers. This requires a copy and paste.
Paste tool no longer works. Use Ctrl-V (CMD-v) to paste directly into the editor window after copying image to the clipboard.
Depending on your browser you may have to click the Turn supplementary image paste support on
checkbox. Required in Chrome but not in Firefox.
In versions of CKEditor prior to 4.5.1, images can be dragged directly from the desktop into the editing window. The paste method described above for 4.5.1 works in these earlier versions as well.