This thread was 6 years old... Software and technology has changed.
That said this isn't going to ever be a feature of VB3 or VB4 and I highly doubt it is a feature you will ever be able to get implemented even if you are willing to pay someone, it would likely require a completely new editor which is going to be cost prohibitive.
The creator of this thread wanted this ability because he didn't want images to break if they were removed later from external sites.
There is a mod now that automatically copes all external linked images to local images as a regular scheduled task. It runs transparent to the user and would be the way to solve the issue the OP was having.
Even if it isn't 100% perfect for your needs it would be a tremendously better starting point then trying to modify the vBulletin editor.
https://vborg.vbsupport.ru/showthread.php?t=253309