Not sure, but you could try grabbing a thread design image that someone else is using and take the dimensions off of that.
I'm currently making a thread design for my new and upcoming service.
I previously used other websites to advertise on, but it seems like the dimensions on MPGH are different than other sites.
So, yeah:
What are the dimensions pixels of a post to ahieve the largest and best quality of a picture(s)?
Not sure, but you could try grabbing a thread design image that someone else is using and take the dimensions off of that.