That's because the forum is converting the images from JPEG to AVIF. Browsers will support AVIF, but support for that format outside of web browsers is pretty spotty, which is why I hate that this conversion is happening.
If you use Windows, then viewing these images requires that
both the HEIC and AV1 extensions are installed:
HEIC:
You must be registered to see the links
AV1:
You must be registered to see the links
Both should be installed by default in Windows 11, but you may need to manually install one or both in Windows 10. And if you're not using Windows, then you need to Google what is needed to view AVIF on your platform.