I found a dirty hack. You change the extension of the picture before dropping in Evernote. It will appear as an external file and then you can change the extension back with "Rename attachment..." and it will work as any other file. You can change the extension on a mac by doing "Get info" on the picture file. Replace the extension after the dot (usually .png or .jpg) by .lol, drag and drop it into evernote, right click + "Rename attachment..." to put back the real extension instead of the .lol Painful but efficient.