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Troubleshooting Pass Thru Mode: Why Some Images Don't Convert to WebP

18 threads Sep 16, 2025 Plugin → Converter for media – optimize images | convert webp & avif

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Many users of the 'Converter for Media – Optimize images | Convert WebP & AVIF' plugin encounter a common issue: they've configured Pass Thru mode, the bulk conversion appears successful, yet some images on their site stubbornly remain in their original format (like JPG or PNG). This can be confusing and frustrating, especially when you're counting on those WebP conversions to improve your site's performance.

This guide explains why this happens and outlines the most effective steps to diagnose and resolve the problem.

Why Pass Thru Mode Might Not Work for All Images

Based on community support threads, the Pass Thru method is an alternative for servers where the recommended .htaccess method cannot be configured, typically on NGINX servers where the user lacks server-level access. However, this method has inherent limitations that can prevent it from working universally:

  • Theme or Plugin Incompatibility: The Pass Thru mode works by rewriting image URLs in the HTML output. Some page builders (like Oxygen Builder) or themes that generate HTML in a non-standard way, or tools that minify HTML after it's processed, can prevent the plugin from finding and rewriting the image sources.
  • External Images: The plugin is designed to convert images from your own server's uploads directory. It will incorrectly try to process images embedded from external domains, resulting in broken image links (e.g., https://your-site.com/wp-content/webpc-passthru.php?src=https://external-site.com/image.jpg).
  • CSS Background Images: Images loaded as CSS backgrounds are often not captured by the HTML URL rewriting process of the Pass Thru method, meaning they will not be converted.
  • Server Caching: Highly aggressive caching configurations can sometimes serve a cached version of the page before the plugin has had a chance to rewrite the URLs.

How to Troubleshoot and Resolve the Issue

If your WebP images are not loading in Pass Thru mode, work through the following steps.

1. Verify the Conversion Actually Worked

First, confirm that WebP versions of your images exist. Navigate to Settings > Converter for Media and run the 'Start Bulk Optimization' process. Once complete, you can use an FTP client or your hosting provider's file manager to check if the /wp-content/uploads-webpc/ directory exists and contains WebP files. If this folder is empty, the conversion failed and you should check your server's error logs for issues with GD or Imagick.

2. Check for Conflicts with Themes and Plugins

This is the most common cause of partial functionality. The 'Converter for Media' team suggests a standard troubleshooting practice:

  1. Temporarily switch to a default WordPress theme (like Twenty Twenty-One).
  2. Clear all your caches (plugin, server, and browser).
  3. Check if WebP images are now loading. If they are, the issue is with your theme.
  4. If the issue persists, disable all other plugins except Converter for Media and check again. If it works, re-enable your plugins one by one to identify the one causing the conflict.

Common conflicts arise with page builders, aggressive minification/optimization plugins, and lazy loading plugins.

3. Confirm It's Not an External Image Issue

Inspect the images that are not converting. Right-click on the image and select 'Open image in new tab'. Look at the URL in the address bar. If the URL points to a different domain, it is an external image and will not be converted by the plugin. The Pass Thru method will generate a broken link if it tries to process it. There is no setting within the plugin to disable conversion for external images; they are simply outside its scope.

4. Consider the Server Configuration (The Best Solution)

The most effective and recommended solution is to not use Pass Thru mode if at all possible. The official guidance from the 'Converter for Media' team is clear: Pass Thru mode is an alternative and will not work with all images. It is also described as a "significantly worse option" because it relies on PHP to rewrite every image request, which is slower than server-level rewrites.

The .htaccess (or NGINX redirect) method is faster, more reliable, and works with all image types, including CSS backgrounds. If you are on an NGINX server and see a "server configuration error," you must contact your hosting provider's support, provide them with the configuration rules generated by the plugin, and ask them to apply them to your server's NGINX configuration file. This is the only way to achieve optimal functionality.

Conclusion

While Pass Thru mode is a helpful fallback, its limitations mean it cannot guarantee full WebP conversion for all sites. The most reliable path is always to configure server-level redirects via .htaccess or NGINX. If you are stuck with Pass Thru mode, your best recourse is methodical troubleshooting to identify conflicts with your theme or other plugins.

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