3. The method of claim 2 , wherein the accepting and displaying at step (a) includes: (a.1) each time a standard character is selected for inclusion in the message entry field via the user interface of the client device, issuing both a local query to the local data store of the client device and a remote query to a remote server to identify unique custom character reference(s) that have a tag word associated with the accepted standard character or a sequence of standard characters including the accepted standard character, if any exist; (a.2) comparing the unique custom character reference(s) that is/are returned in response to the local query to the unique custom character reference(s) that is/are returned in response to the remote query to thereby identify which custom character image data is not yet stored in the local data store of the client device; (a.3) obtaining, from the local data store of the client device, the image data for each of the unique custom character reference(s) identified in response to the local query; (a.4) obtaining, from the remote server, and saving in the local data store of the client device, the image data for each of the unique custom character reference(s) identified at (a.2) as not yet having been stored in the local data store of the client device; and (a.5) displaying, in a custom character tray, the one or more custom characters for which the image data is obtained from the local data store of the client device and from the remote server.
2. The method of claim 1 , further comprising storing the following within the local data store of the client device: a unique custom character reference for each of a plurality of custom characters; image data for each of the plurality of custom characters for which the unique custom character reference is stored; and one or more tag words for each of the plurality of custom characters for which the unique custom character reference is stored.