When using Amazon as a source for cover images, images within the shelf browse feature can be 1x1 pixel, thus preventing the no image available image from showing. An example of the image returned is: https://images-na.ssl-images-amazon.com/images/P/0844254525.01._AA75_PU_PU-5_.jpg Some would prefer the "no cover image available" to be shown rather than this 1x1 pixel image.
Amazon always returns a 1x1 pixel if they don't have a cover image. That's why we have some JavaScript which is supposed to check for them: /koha-tmpl/opac-tmpl/bootstrap/js/amazonimages.js This technique originated here: http://archive.oreilly.com/pub/a/javascript/2003/10/21/amazonhacks.html It sounds like the problem is that either the script isn't running correctly in the shelf browser or the shelf browser is using markup which prevents the images from being recognized by the script.
This bug doesn't appear to be valid in master.