1. Search for "fbs.ImageResizer.InterceptModule" and replace it with "ImageResizer.InterceptModule". There should be 1 or 2 occurrences, in the <modules> and <httpModules> sections. It should look like this:

    <modules>
        <add name="ImageResizingModule" type="ImageResizer.InterceptModule"/>
    </modules>
   

2. At the top of the file, you should see a <configSettings> section. If not, add it, and then add <section name="resizer" type="ImageResizer.ResizerSection"/>. It should look like this:

    <?xml version="1.0"?>
    <configuration>
        <configSections>
            <section name="resizer" type="ImageResizer.ResizerSection"/>
    </configSections>
   

3. Add a <resizer> section after </configSections>:

    ...
    </configSections>
    <resizer>
        <!-- This configuration mostly duplicates v2 default behavior-->
   		
        <pipeline fakeExtensions=".ashx" vppUsage="Fallback" /> <!-- Optional, these are the defaults anyway -->
   		
        <clientcache minutes="1440" /> <!-- defaults to 0, no expires header sent. -->
   		
        <sizelimits imageWidth="1680" imageHeight="1680" totalWidth="3200" totalHeight="3200" />
   			
        <diskcache dir="imagecache" subfolders="32"
                    enabled="true" autoClean="true" hashModifiedDate="false" /> 
   					
        <!-- defaults to  autoClean="false" and hashModifiedDate="true"
                 hashModifiedDate=true causes new files to be written when the source files change, but also prevents 
               locking delays that can occur when trying to update a cached file on a busy site-->
   				   
        <plugins>
            <add name="DiskCache" />
            <add name="PrettyGifs" />
            <add name="DropShadow" /> <!-- Only if you use the drop shadow feature -->
            <add name="AnimatedGifs" /> <!-- Only if you resize animated Gifs -->
            <add name="FolderResizeSyntax" /> <!-- Only if you use the /resize(x,y,f)/ folder syntax -->
        </plugins>
    </resizer>
   

4. Visit /resizer.debug.ashx to make sure everything is working.

Setting-by-setting explanation

• ImageDiskCacheDir is now <diskcache dir="value" /> (defaults to "imagecache")
• ImageCacheSubfolders is now <diskcache subfolders="value" /> (defaults to 32)
• DisableCacheCleanup is now <diskcache autoClean="false" /> (defaults to false, no cache cleanup)
• DiskCacheAlwaysInvalid=true is the equivalent of <diskcache enabled="false" /> (defaults to true, enabled).
• ImageResizerMaxWidth and ImageResizerMaxHeight are now <sizelimits imageWidth="value" imageHeight="value" /> (See SizeLimits documentation for detailed info)
• ImageResizerClientCacheMinutes is now <clientcache minutes="value" /> Defaults to 0 instead of 1440.
• DisableCustomQuantization=true is the same as removing <add name="PrettyGifs" /> from the <plugins /> section.
• ResizeExtension is now <pipeline fakeExtensions=".ashx,.axd" />. Multiple extensions can be separated by commas. Defaults to ".ashx", the safest choice.
• MaxCachedImages has been replaced by a more flexible system of cleanup criteria. Basically defaults to 12,800.
• DisableImageURLAuthorization no longer exists, but can be implemented by handling the Pipeline.AuthorizeImage event and setting AllowAccess to true.
• ImageResizerUseVirtualPathProvider and ImageResizerUseVirtualPathProviderAsFallback have been replaced by <pipeline vppMode="Fallback|Never|Always"" />. Defaults to "Fallback".
• AllowURLRewriting has been removed. URL rewriting is permitted by default, but URL rewriting is also not performed by default, except for removal of fakeExtensions (".jpg.ashx" -> ".jpg").
• The /resize(x,y,f)/ syntax has been moved to a plugin, FolderResizeSyntax.