app_offline.htm is a special static html page in ASP.NET, which we only copy to IIS root folder when we need to bring down a web site for maintenance. app_offline.htm works for SharePoint too.
1. Create an html page with the name “app_offline.htm”.
2. Make sure file size is more than 512 bytes.
3. Copy to the IIS Root folder.
As long as “app_offline.htm”. file exists in the IIS root, no requests will be processed. Trying to access any page will output app_offline.htm content.
In order to end the maintenance activity, just delete app_offline.htm in IIS root folder.
Adding Images to app_offline.htm
In order to add an image to the app_offline.htm, use base64 encoding of the image. (Convert any image into a base64 string here)
Note: However I experienced that big images were not rendering. There were no errors but image just not loading.