SSL or Clear based on URI

You'll need a serverside SSL profile as well. Then, just disable the SSL for the appropriate URI:

when HTTP_REQUEST {
  set usessl 1
  if { [string tolower [HTTP::uri]] ends_with "one/happy.gif" } {
    pool http_servers
    set usessl 0
  }
}
when SERVER_CONNECTED {
    if { $usessl == 0 } {
      SSL::disable
    }
}  

This rule assumes your https server pool is set as the default on the profile. If that's not the case, you'll need to set it in the rule.

For greater detail, there is a more comprehensive post here:

http://devcentral.f5.com/Default.aspx?tabid=28&view=topic&forumid=5&postid=1635