Image Extention .tif
You can add file types in Safe File Extension Options in the admin console. (I don't know if it's advised, but it's certainly possible :)

Go to Administer > System Settings > Safe File Extensions, then click 'Add Safe File Extension'. Enter the file extension (without the dot) in the Label field, make sure it's enabled, then save.

Edit: However, this doesn't actually help with .tif files. TIFFs aren't meant for web use, so (presumably) the image processing in CiviCRM or the CMS isn't designed to deal with TIFF files - it looks like CiviCRM has an additional check to stop the upload because of this. There are probably modules for Drupal that will add this capacity (Imagemagick comes to mind), but I don't know if they'll be picked up by CiviCRM.

If you don't need the images to be TIFF files, I'd recommend using a progamme like Irfanview ( http://www.irfanview.com/ ) to convert them to jpgs.

If you do need to have the files as TIFFs, then add a custom file field for the contact record - this will allow you to upload the files, but won't display a thumbnail. (You can always upload a jpg version as well to get the thumbnail).