Haven't tried it but this guy seems to know what he's talking about.Prerequisites:
1. Install OpenSSH through cydia
2a. Windows: Search google for "putty", download and save to desktop.
2b. Mac: Open terminal
3. Get IP address from phone. If SBSettings, it's listed there, otherwise Settings->Wifi
1. Double click putty icon on desktop
2. Enter IP address from above where it says Host name
3. Wait a few seconds. It takes a while the first time. You will be prompted to accept the SSH key. Click yes.
4. Login as root
5. password is alpine
1. Open a terminal
2. Type "ssh root@IP Address from above"
3. Password is alpine
Once in phone:
1. cd /var/stash/Themes and press the tab key, then enter
2. ls -l, note the name of your theme in the list
3. cd <your theme name>.theme
4. cd Icons
5. cp biteSMS.png biteSMS.png.bak
6. cp /var/stash/Applications/MobileSMS.app/icon.png biteSMS.png
Respring your phone either through SBSettings, Winterboard or reboot.
Thanks,this is what i did using iFile...
backed up Icon-Small@2x.png and Lock-screen-QR@2x.png (renamed them with .bak at end)
pasted TWICE into /var/stash/applications/biteSMS.app/
renaming the second one to Lock-screen-QR@2x.png