ImageMagick convert works from command line on server, but not in drupal: no decode delegate for this image format

I have just installed ImageMagick on Ubuntu 14.04, it runs fine from the command line. I then installed the ImageMagick module in Drupal 7.32 and chose the ImageMagick option in Image toolkit. Then I tried to view the thumbnail image style and got the following errors:

User error: ImageMagick error 1: convert.im6: no decode delegate for this image format `/opt/bitnami/apps/drupal/htdocs/modules/image/sample.png' @ error/constitute.c/ReadImage/544. convert.im6: no images defined `/opt/bitnami/apps/drupal/htdocs/sites/default/files/styles/thumbnail/public/modules/image/sample.png' @ error/convert.c/ConvertImageCommand/3044. in _imagemagick_convert_exec() (line 519 of /opt/bitnami/apps/drupal/htdocs/sites/all/modules/imagemagick/imagemagick.module).
Warning: Division by zero in theme_image_style_preview() (line 804 of /opt/bitnami/apps/drupal/htdocs/modules/image/image.admin.inc).
Warning: array_intersect_key(): Argument #1 is not an array in theme_image_style_preview() (line 806 of /opt/bitnami/apps/drupal/htdocs/modules/image/image.admin.inc).

The sample.png original is visible but not the thumbnail version. Permissions on my files directory are drwxrwx--- and the apache user is in the same group.

The png delegate is available - remember this works from the command line with png and jpg.

Any ideas what could be causing this?

ImageMagick version is 6.7.7-10