See also T135430.
The added border should increase the layout box of the image by 2 pixels. However in an effort to avoid layout differences the image layout was given box-sizing: border-box; (which prevents the increase in layout size) and overflow: hidden; (which hides the excess image data of the right and bottom edge).
This however effectively deforms the image which is wrong.
This bug is currently invisible due to the image not being inserted until after it completes loading, at which point the border is hidden by the loaded class. However after T135430 is fixed, this will likely become an issue.