I personally don't use the HTC dialer. I use the default dialer that comes with WM 6.1.....And, since I do that, I just copy whatever wallpaper I dig into the two files: dlrback_land.png and dlrback_port.png. Then, I take all the files and copy them in the windows directory on my device. After you copy them, reboot your device.
If you are currently using the HTC dialer, you need to modify your registry....
HKLM\Security\Phone\Skin\Enabled. Change setting to 0 to disable and reboot.
Here are a couple I have....
