Open the mail application
Create a new message
Open the browser (Iceweasel or whatever)
position the mouse over the picture and drag it over the message window
Now, the right workflow for opening it with the GIMP:
Open the GIMP
drag the image over it
right click the image, then select "open with" and then "Image editor (the GIMP)".
You don't need to save the image to disk at all.
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