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Shmuff Finally someone answers to you. Sorry, I've been too lazy to do it these past days.
Right, the first thing you need is the "Url" of an image, thereby you need it to be on the Internet (I actually have an account in Deviantart to post the stupid things I do so I can get the "url"). Once you have it on the Internet, you right-click it, press "Copy the image link" or whatever that appears (not the image per se, but the link) and then, when in Mangadex, press the tab with an image that is in the above line, the 8th one starting from the left. Then paste the link between the square brackets (don't add any spaces).
Today's probably the day I'll discover I've been doing it wrong for years.