I am having the same issue. I looked at your regedit fix, but I don't have Outlook there. I only have Hotmail and Windows Mail.
I am using the Send to email command from my pictures folder. I have tried setting the default for gmail app and chrome and I have set the protocol file type setting for gmail.
Nothing seems to work. I still get the same message.
Unfortunately, I lost my notes on this matter. I cannot remember how I fixed it. If you could copy and paste my original note I may be able to back track and figure it out. My problem was I was having problems with outlook 2007, which i corrected. I have since had to send laptop in for warranty work and have not attempted to load 2007 up again.