In Options/Preferences/Editing (tab), what is the setting for "New still image length (seconds)"? The default should be 5 seconds.
Otherwise, when zooming in on the one frame on the timeline, can you lengthen the event by dragging the RH frame edge?
If there's still a problem, it would be helpful if you would upload a detailed MediaInfo (a free app) report on the image media. More info about MediaInfo here which includes a link to the app.