How are you doing it? Are you using Cisco Phone Designer, or manual tftp method? Call Manager? What version?
You have to make sure the List.xml file is in the proper directory, and make sure the image files are in the proper directory and that the tftp server has been restarted.
7965 and 7945 background requirements
http://www.cisco.com/en/US/docs/voice_ip_comm/cuipph/7965g_7945g/6_1/english/administration/guide/7965cst.html#wp1088046 Thumbnail = 80x53 16bit color (65535 colors)
Full Size = 320x212 16bit color (65535 colors)
Directory = /Desktops/320x212x16
List.xml file=
<CiscoIPPhoneImageList>
<ImageItem Image="T
FTP:Desktops/320x212x16/THUMBNAIL.png"
URL="T
FTP:Desktops/320x212x16/FULLSIZE.png"/>
</CiscoIPPhoneImageList>