This issue is that the b/w images are not being generated, those images are created when you upload the original ones, So, why the b/w images are not being generated?
This happen because 2 major reasons:
1.- The upload folder has not write permissions, set your permission to 755 using your FTP client.
2.- The server has not the libraries for created the b/w images, this libraries are GD for PHP (http://en.wikipedia.org/wiki/GD_Graphics_Library) all the decent servers around the net has those libraries by default, is a common library.
Please contact your hosting service to asking for those libraries.