There doesn't seem to be an easy to perform this feat. Albeit I found this online:
"You need to open the (ROM).zip that you are using and edit an .apk and then use ADB to push it back to the phone.
I'm using Virtuous Unity 1.3.1. I've done it successfully on a few other ROMs too.
You'll need a graphics editor (I used MS Paint), Android SDK with adb, 7-Zip.
Here's how I did it...
As always, MAKE A BACKUP! Its not my fault if something goes wrong.
1 - Open the .zip copy of the ROM that you're using
2 - Extract a copy of /system/app/Phone.apk
3 - Open Phone.apk using 7-zip and copy the following files to your desktop (don't close the 7-Zip window):
-phone_default_270.png
-phone_default_300.png
-phone_default_incall_270.png
-phone_default_incall_300.png
4 - Find a picture that you want to use and resize it to 300x300 and 270x270. Save it as the above file names.
5 - Copy the 4 files back into the Phone.apk archive and exit 7-Zip
6 - Copy the new Phone.apk file into your adb directory (Android SDK, usually in the platform-tools folder)
7 - Connect your phone to your computer and ensure its in charge-only mode with USB debugging enabled
8 - Open a command prompt window, change to your adb directory, and run the following commands:
adb devices {to make sure the device is connected}
adb remount {sets it to read/write}
adb push Phone.apk /system/app/Phone.apk {replaces the Phone.apk file}
9 - Disconnect your device and reboot
10 - Enjoy!"
Compliments of a gentlemen called Kyle