Need help with iRule using iFiles

I'm guessing you are trying to match the file no matter the path. If that's the case you need a glob added to the switch and "*" added to your jpg and gif uris:

when HTTP_REQUEST priority 10 { 
    switch -glob [HTTP::uri] {
        "*/down.jpg" { 
            HTTP::respond 200 content [ifile get down.jpg] "Content-Type" "image/png" "Connection" "Close"
            event HTTP_REQUEST disable
        }
        "*/poweredby.gif" { 
            HTTP::respond 200 content [ifile get poweredby.gif] "Content-Type" "text/css" "Connection" "Close"
            event HTTP_REQUEST disable
        }
        default {   
            if { ( [HTTP::uri] starts_with "/"  ) } {
                HTTP::respond 200 content [ifile get down.html] "Content-Type" "text/html" "Connection" "Close"
                event HTTP_REQUEST disable
            }
        }
    }
}