Narrow images like the one below have an explicit width set on them that causes them to look a bit poo. The issue appears to only occur when actually posted—the image looks fine right now in the post preview.
The post I originally saw this issue on appears to have resolved somehow, so here’s a screenshot of what I’m currently seeing in this post.
You can see what happened in the editor and why it no longer looks bad. The system edited your post and stored a local copy of the image you were linking. And since the image is that long, the system cropped its preview.