Here is a link to download aerial photo data for Montana, MrSID format with a world file. displays fine in ArcMap, but not in ArcGIS Explorer. Should be able to use this directly as a raster file in Explorer.
Here is a link http://www.nris.mt.gov/nsdi/orthophotos/naip_2009.asp?MapNo=36
Aerial photos are in Montana State Plane Nad83, I have explorer in the same datum.