Kiet Huynh

This plugin will help to replace broken images in posts and pages with a default image.

Replace Broken Images

It's OK

Alternate image with a default image if source image is not found on posts and pages.

ktsvetkov

This plugin is designed to “fight” the broken UTF8 
characters on your WordPress powered site.
Install the UTF8 Sanitize WordPress plugin and you 
will be able to fight those ugly characters. Additionally, you can customize 
what actions you want to perform on them – either convert them to their original 
values once and for all by modifing your posts’ database, OR leave all 
your posts intact and instead convert the ugly chars each time a post is loaded 
(all these options are controlled from the WordPress admin panel).

UTF8 Sanitize

This plugin is designed to handle the broken characters that might appear on your blog posts, if their content is converted from/to UTF8 and ISO-8859- …

Replace Broken Images vs UTF8 Sanitize