To follow up on Paul's post, Evernote will do it in one tap (not including the camera shutter) if you're on Android. Add the widget, add the "quick image" tab to it, and you can tap that to quickly add a photo.
Like johnaohman said, though Dropbox is even faster. I use Google+ (and by extension, Picasa) which has the same auto-upload feature. Quite handy.