WebP plugins

With slimmage.js, you can use WebP for supporting browsers - without breaking the others. Slimmage makes responsive images easy to implement - both client and true image size is controlled with css max-width properties.

We have a guide for using Slimmage and ImageResizer together.


  1. Either run Install-Package ImageResizer.Plugins.WebP in the NuGet package manager, or add ImageResizer.Plugins.WebP.dll to your project.

  2. Add <add name="WebPEncoder" /> and/or <add name="WebPDecoder" /> inside <resizer><plugins></plugins></resizer> in Web.config.


Simply reference a .webp file as you would a .jpg


A 100px wide jpeg will be returned.

If the extension is not .webp, you can add &decoder=webp to force webp decoding first, instead of waiting for the fallback path.


Add &format=webp to any URL to encode the result in webp format instead of jpg/png


  • Quality=1..100
  • Lossless=true/false (defaults false)
  • NoAlpha=true/false (defaults false)

Rule of thumb for converting jpeg quality values to webp

In general, webp achieves the same visual quality with a much lower quality parameter.

The first value is the jpeg quality, second is webp quality for same visual clarity.

  • 90->78
  • 80->65
  • 70->55
  • 50->40
  • 40->30
  • 20->10
  • 10->3
  • 5->0

HTTP Error 404.3 - Not Found

The page you are requesting cannot be served because of the extension configuration. If the page is a script, add a handler. If the file should be downloaded, add a MIME map.

When you get this error, you'll need to add a mime-type mapping in web.config

     <mimeMap fileExtension=".webp" mimeType="image/webp" />

This plugin (WebP plugins) is part of the Elite Edition

Where is the plugins section?

The <plugins> section is located in Web.config, and is nested inside the <resizer> element, which is nested inside <configuration>. For examples, see this sample Web.config file.

Where can I find the dll?

We prefer that you install via NuGet, but you can also find the plugin DLL files in the /dlls/release folder of your download.

How do I typically install a plugin via Web.Config?

  1. In Visual Studio, right click on your project and choose "Add reference". Browse to the plugin DLL and click "OK".
  2. In the <plugins> section of Web.config, insert <add name="PluginName" />
  3. Look at the plugin documentation to see what configuration options (if any) are available.

How do I typically install a plugin via code?