This allows you to insert a list of pictures into the timeline. You specify
the base file name and MainActor will search for files with sequential numbering.
For example, the names of a set of files might be pic001.gif, pic002.gif,
pic003.gif and so on. You can specify what the duration of each image will be on
the timeline, so that a setting of 00:00:01.00 would mean that each picture would
cover one second on the timeline. The pictures are loaded as a group and
treated as a single object on the timeline.