You can click on that image to open it in full resolution, and when it does, it will show the sharp corners, not rounded corners. Like u/Redpilled2Depression said, it's just how Twitter/X display the embedded pictures with rounded corners then once you click on the image, it will show sharp corners once displayed in full resolution.

Same as when you look at an user's profile picture, it's a circled image, but if you open the image in a new tab, it will show the full image, which will be a square and showing some more of the picture that the circle overlapped.