Alex, I had the same issue and it was not fixed but the latest plugin update. I am getting:
Could not find /mnt/r1234/d12/s12/b123456/www/pitstopmedia.com/sem/images//mnt/w1234/d12/s12/b123456/www/pitstopmedia.com/sem/images/buy-now-and-add-to-cart-next-to-each-other.png
The bold part was added by Smushit somehow. The image exists at http://www.pitstopmedia.com/sem/images/buy-now-and-add-to-cart-next-to-each-other.png
