Suppose you have an image with size 300x450. Highslide works fine. Now you replace this image with a larger one, say 400x600 (with the same file name). However, everytime you click on the thumbnail, Highslide remembers the old image size.
The fix is to set hs.flushImgSize to true, refresh the page and open the expanding image. Now remember to remove hs.flushImgSize again, or your server will get a lot of requests for a file called 'null'.