People need to calm down and read up on fair use. Also see the following advice from Gemini. I'm not a lawyer so take from this what you will. And if Disney issue a takedown notice, just comply.
Recommendations:
* Make it Distinctive: To minimize risk, try to make your character and its style significantly different from existing Muppets. Avoid directly copying recognizable features.
* Avoid Using the Term "Muppet": Don't explicitly call your creation a "Muppet" or use the Muppets trademark in your description.
* Clearly State It's Fan-Created: If your image could be interpreted as being officially related to the Muppets, explicitly state that it is fan-created and not affiliated with Disney.
* Be Prepared for a Takedown Request: Even if you believe your use is fair, Disney could still send a takedown request to the forum. You would then need to decide whether to comply or challenge it (which can have legal implications).
In conclusion, while the risk of legal action for a non-profit, original image on a small forum is likely low, technically it could still be considered copyright infringement if it draws too heavily on the established Muppet style. Creating a distinctly different character and avoiding the use of the "Muppet" trademark will help to minimize any potential issues.