If I understand this correctly, your MAT files on her Mac want to see PICT files for textures, but couldn't find or read your TIFF files, but the files appeared to be PICT files when she inspected them? Mac OSX may interpret TIFF files as PICT so that it can open them in the Preview app that is part of OSX, but the files are not really converted to PICT filesjust labeled as such. It is also possible that her download/decompression software is set to label TIFF files as PICT without actually converting them. IIRC, P5 on the Mac wants to find PICT or JPG files for texture maps. The easiest way to prevent this in the future is to tell Mac users to open the texture files first in Photoshop and save them as PICT. The nicest way would be to do the conversion yourself: just "save as" twice, once as a TIFF file and once as a PICT or JPG.