Mediawiki Error creating thumbnail: sh: convert: No such file or directory

The error below should be referring to a non existent ImageMagic executable:

Error creating thumbnail: sh: /usr/local/bin/convert: No such file or directory

After installing the executable my MediaWiki installation was still complaining. I fixed the problem using action=purge on the page, for example:

http://wiki.nestorurquiza.com/index.php/Nestor?action=purge