According to the makers of FileJuicer (which can perform this conversion on a Mac),
The ithmb files are compressed using 16 bits per pixel in YUV format for television and they are therefore a bit smaller than the converted TIFF files.
It appears that the ithmb format varies from device to device (iPod Photo, iPhone, etc.), so you'll probably have to manipulate things based on which device the file came from.
FileJuicer ithmb Format Page:
http://echoone.com/filejuicer/formats/ithmb
Wikipedia YUV Page:
http://en.wikipedia.org/wiki/YUV
FOURCC YUV/RGB File Information:
http://www.fourcc.org/fccyvrgb.php
http://www.fourcc.org/yuv.php