Answer
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
If the message of the cartoon phone wallpaper is sublime and ethical, you can go for it. If the message is obscene or unethical, then you ought to avoid it.
In responding to your question, Sheikh Ahmad Kutty, a senior lecturer and an Islamic scholar at the Islamic Institute of Toronto, Ontario, Canada, states:
It all depends on the theme or message conveyed through the cartoon phone wallpaper. If the message of the cartoon phone wallpaper is sublime and ethical, you can go for it. If it is obscene or unethical, then you ought to avoid it.
We are accountable for the things we project or represent, or communicate.
Almighty Allah knows best.