Using F5 as a fall back host

That's close. The /error/image.png only exists in the HTML you're sending to the client from the iRule. Can you give this a try?

I added some logging so you can check the /var/log/ltm log file for debug info if it doesn't work. You can check the file on the CLI using tail -f /var/log/ltm, (ctrl+c to stop tailing the file), or in the GUI under System >> Logs >> Local Traffic. Make sure to comment out the debug statements once you're done testing.

when RULE_INIT {
   set ::polr_html {All Servers Down}
}
when HTTP_REQUEST {
   log local0. "Received request from [IP::client_addr] to [HTTP::host][HTTP::uri], with [active_members myPool]"
    Check if the pool has any active members
   if {[active_members myPool] == 0}{
      log local0. "No active members"
       The error condition was true, so respond back to client from the iRule
      switch [string tolower [HTTP::path]] {
         /error/image.png {
            log local0. "Request was for /error/image.png"
            HTTP::respond 200 content [b64decode [lindex $::logo_png_class 0]] "Content-Type" "image/png"
         }
         default {
            log local0. "Request was not for the image, so we're sending the HTML"
            HTTP::respond 200 content $::polr_html
         }
      }
   }
}

Aaron