As of the time of this writing, there are a couple issues with the "Cisco Unified IP Phone 7975G, 7971G-GE, 7970G, 7965G, and 7945G Administration Guide for Cisco Unified Communications Manager 8.6 (SCCP and SIP).pdf" instructions on how to implement custom wallpapers on 7945G guides.
1. The correct image size for 7945G's is 320x212, not 320 by 216 as indicated in the guide.
Therefore the correct directory path that has to be used or created is "Desktops/320x212x16".
The PNG formated pictures must also be sized as 320x212 for the wallpaper, and 80x53 for the thumbnail.
The List.xml file should also reference the correct directory / size, like below:
<CiscoIPPhoneImageList>
<ImageItem Image="TFTP:Desktops/320x212x16/YourWallpaper-TN.png"
URL="TFTP:Desktops/320x212x16/YourWallpaper.png"/>
</CiscoIPPhoneImageList>
2. If you copy and edit the List.xml sample from the PDF for use in production and do not validate using an XML aware text editor (i.e. you use MS Notepad, rather than Notepad++, etc.), you will likely be including some extended characters in the XML file rather than "real" double qoutes around the directories. This will prevent your phones from accessing the wallpaper's as well. Check your XML file in something other than MS Notepad to verify your double quotes are really double qoutes and that the XML is valid.