What happens when you right-click on the image service layer and choose Refresh? Or, click on the Manage Layers button, choose your image service layer and then hit the Clear Cache button. Or, click on the orb in the upper left-hand corner of the application, click on the ArcGIS Explorer Options button at the bottom of the dialog window, click on the Advanced button in the TOC on the left side, click on the Delete All Caches button in the middle of the window under Cache Management options.
I tried to use this map http://www.arcgis.com/home/item.html?id=b71e06bddd5d448382cc3732299e9e36 to reproduce the problem.
Do you see the same problem when you use this map?
Mark