iRule to replace http with https for specific URLs

Try something like this:

when HTTP_RESPONSE {
  STREAM::disable
  set strexp ""
  if { [HTTP::header Content-Type] matches_glob "text/*ml" } {
    append strexp "@http://user.example.net/images@@"
    append strexp "@http://user.example.net/port@@"    
  }
  if { $strexp ne "" } {
    STREAM::expression $strexp
    STREAM::enable
  }
  if { [HTTP::header exists "Location"] } {
    if { [HTTP::header "Location"] starts_with "http://" } {
      HTTP::replace "Location" [string map "http:// https://" [HTTP::header "Location"]]
    }
  }
}
when STREAM_MATCHED {
  switch -glob [string tolower [STREAM::match]] {
    "http://user.example.net/images" { STREAM::replace "https://user.example.net/images" }
    "http://user.example.net/port" { STREAM::replace "https://user.example.net/port" }
  }
}