it'd be simple enough just to use data: urls for those.

uparrow =

  data:image/gif;base64,R0lGODlhCgAKALMJANPT06
  enp%2Fb29r%2B%2Fv52dnfn5%2Bbq6usLCwpqamv%2F%
  2F%2FwAAAAAAAAAAAAAAAAAAAAAAACH5BAEAAAkALAAA
  AAAKAAoAAAQcMMlJq712GIzQDV1QFV0nUETZTYDaAdIhz3ISAQA7

It is fairly stupid that HN doesn't use an expires header to cache the upmod/downmod arrows, so everyone requests them every time the visit.