TIFF is only a container format (like AVI on the video). The file extension does not actually signify how the image data inside is encoded. Most professional photo editing applications create their own flavour of TIFF which is unreadable in other software. PIL might support only certain subset of TIFF flavours (e.g. uncompressed).
If your system is a web upload style system I suggest you stop accepting TIFF format to avoid problems altogether.
More information: http://en.wikipedia.org/wiki/Tagged_Image_File_Format