When you attach the photo and save a copy on the forum server (preferred method), then the pics display in a reduced format when viewed on a PC. If you link to the original then you'll often display in the largest format that the file was saved or maximum that the screen allows.
If you're running off a small screen you won't necessarily notice the difference.
It's a good idea to retrieve the file and reference locally as all too often things get moved on the interweb and then all your hard work gets lost due to broken links.
Just go on vortex and look at threads from a few years ago and half the pictures are missing
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