Manual:$wgThumbnailScriptPath

From MediaWiki.org

Thumbnail settings: $wgThumbnailScriptPath
Give a path here to use thumb.php for thumbnail generation on client request, instead of generating them on render and outputting a static URL.
Introduced in version:	1.5.0
Removed in version:	still in use
Allowed Values:
Default Value:	false

Other settings: Alphabetical | By Function

[edit] Details

Give a path here to use thumb.php for thumbnail generation on client request, instead of generating them on render and outputting a static URL. This is necessary if some of your apache servers don't have read/write access to the thumbnail path. Example:

$wgThumbnailScriptPath = "{$wgScriptPath}/thumb.php";