Some peers, in particular older phones, have problems when being sent lots of photo data. It may be more useful to exclude photo data altogether for such peers.
See "[SyncEvolution] Sync failure due to large addressbook entires."

A possible solution would be:
- introduce a new session variable in the <sessioninitscript>
- set that variable in remote rules (see stripUID for an example)
  or based on a new config property (see preventSlowSync)
- extend the <outgoingscript> for contacts such that it
  unsets the PHOTO property before encoding a contact for the peer
  if that session variable is set - see VCARD_OUTGOING_PHOTO_INLINING_SCRIPT

That should take care of sending data to the phone. Ensuring that the photo doesn't get lost when receiving an update is harder. Something might be possible with a custom <mergescript>, but I am not sure whether it really gets invoked during a normal import.

Alternatively, dynamically updating the field attributes after parsing the peer's CtCap would be a more elegant solution, because it would use existing libsynthesis mechanisms:
- don't send properties to a peer which the peer doesn't support
  (strip PHOTO when sending)
- preserve local properties not supported by the peer (preserve PHOTO)

However, implementing that will require considerable changes in libsynthesis:
- expose the field list attributes to scripts
- ensure that a script is called after parsing CtCap and before using the
  field list (perhaps <datastoreinitscript> is suitable)