Computer-readable media having corresponding apparatus embodies instructions executable by a computer to perform a method comprising: receiving, from a user interface, an annotation associated with a background image; adding the annotation to a queue of pending annotations; causing transmission of the annotation to a server; removing the annotation from the queue of pending annotations, and adding the annotation to a list of acknowledged annotations, when an acknowledgment of the annotation is received from the server; and generating a display image comprising the background image, annotations in the list of acknowledged annotations, and annotations in the queue of pending annotations.