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File > Export...
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Right click on the image then Export > Export...
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Use a shortcut: Ctrl-Shift-E
All of these will bring up the Export dialog where choices can be made about export size, file type, watermark etc. but option 3 is by far the quickest, three buttons close together.
Then to further speed up your exports this shortcut will help.
Ctrl+Alt+Shift+E [Export with Previous] will export further images with the same settings as the last one you set.
All you Mac users will of course know by now to use Command instead of Ctrl.