This is a 'them' problem and not an 'us' problem.
An alternative (to use at least until Postimg can fix their problem) is to use the ImgBB site - at https://imgbb.com/upload - where you can upload a picture and then from the dropdown set of options choose 'BBCode full linked' to copy the link code. You don't need an account with them to do this but note that file sizes are limited to 32 MB which shouldn't be a problem.
This will create a piece of code similar to :
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[url=https://ibb.co/K7b5jk5][img]https://i.ibb.co/pZb1v81/first-christmas-card.jpg[/img][/url]
The opening and closing url tags in there are not really needed so you could edit this down to :
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[img]https://i.ibb.co/pZb1v81/first-christmas-card.jpg[/img]
Example: