I recently ran into an issue on a wordpress site I maintain. Images randomly stopped showing up on the site.
On further investigation I found that the images were just returning a 404 error.
After some searching the internet, I came across this article which basically said that it had something to do with the plugin ‘User Access Manager’.
The plugin adds a .htaccess file to the wp-content/uploads folder which stopped images being retrieved correctly.
The .htaccess file contains the following code

RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]
RewriteRule (.*) /index.php?uamfiletype=attachment&uamgetfile=$1 [L]