I had a similar problem. I add a georeferenced tiff image to ArcMap, no problem. It's exactly where it should be. I add the very same image to ArcGIS Explorer and it's nowhere close. Overlays half of the continental US, when it should be just a tiny portion of southern Maryland.
I've discovered that the issue in my case was the projection the data used. The raster image had been georeferenced to State Plane, Maryland, units are feet. But the standard Maryland State Plane projection units are meters. I don't believe that AGX is recognizing a custom projection. Once I set my data frame to the more standard projection, and then exported the .tiff to that new projection, the new image worked great!
I wonder if the Projection Expansion pack that you can download with AGX would fix this. That's my next task. I'll let the group know if that works.
~Ellen