I have an issue with my lightbox images being stretched when viewed, despite the width and height being defined in the images.
I have tested it in multiple themes, and it's been awhile since I've been to this page/updated it, so I honestly can't tell you when it started, but it was originally not like that.
I use Simple Lightbox for the images to display them. And I'm thinking it has something to with maybe a recent update to the plugin as it wasn't always like this. I'm using the Beta, but will try to find an older version of the plugin to see if that takes care of it.