Arc 9.1 seems to have trouble with images quite regularly. I don't know if this will help you, but I've installed the "Raster Patch" and it cleared up all the issues I had with .tif files at work. Here is the link if you want to download it: http://support.esri.com/index.cfm?fa=downloads.patchesServicePacks.viewPatch&PID=43&MetaID=1056
Here are the issues it repairs:
In reply to:
CQ00272548 - Support for CCITT G4 TIFF and PackBits TIFF compression types are restored.
CQ00272575 - World files are no longer deleted when manipulating any part of a raster's spatial reference.
CQ00272826 - Support for GeoTIFF tags containing spatial reference information is restored.
CQ00274549 - Support of 16 bit NITF version 2.0 raster files is restored.
CQ00272147 - MrSID Encoder is not creating the associated worldfile (.sdw) when encoding MrSIDs from your input raster datasets.