I ran a program that can analyze all metadata embedded in a picture, called MetaEditor.
Each of my TIF photo files have metadata containing the descriptions, but they are not readable by Media Center. They are, however, read by other software, including MetaEditor, Photoshop and Windows 7 Explorer.
The tags are:
EXIF:
Tag: 270
Name: Image.ImageDescription
Type: String
Value: "this is my description that is not read by MC"
(TIFF tag 270, 0x10E. Description of the image. Note: This property is stored in XMP as dc:description.)
IPTC:
Tag: 2:120
Name: Caption
Type: String
Value: "this is my description that is not read by MC"
XMP:
Tag: dc:description
Name: dc:description
Type: String
Value: "this is my description that is not read by MC"