Is it mistake by stackoverflow or there is bug in my browser
The "Insert an image" modal dialog appears to not be responsive when it comes to browser resizing. Here's a screenshot from Chrome:
The "Insert hyperlink" modal does respond to resizes though. It stays centered (both vertically and horizontally):
So I would guess that this is a bug with the "Insert an image" dialog.
The reason for this is the CSS applied to the image dialog:
Compared to that of the link dialog:
You can see that the link dialog is positioned with percentages, and the image dialog is positioned with pixel values, causing it to stay where it is when you resize the browser window.