Have you made any changes to the functions.php? The size of the slideshow is actually 610 width, so each image is already getting stretched a tiny bit to fit the bigger width. For some reason, the logo image is then using 600x340 dimensions for its size. I would first check your functions.php and see if you have any image sizes set there for that size. If so, change it to W: 610 H: 930.
If not, you can try regenerating the thumbnail for the logo image with a plugin: http://wordpress.org/extend/plugins/regenerate-thumbnails/
OR, if you want to just be done with it ASAP, upload the proper sized image via ftp to overwrite this file: http://www.ckproductions.co.uk/wp-content/uploads/2012/03/skp_logo1-600x340.jpg
I'm not sure what the root cause of it is, but it sounds like something has changed to make the slideshow images a different size or to be cropped. So hopefully the above helps find the solution.