There's a lot of people on the oekaki boards. This can create confusion and a lot of emotional misunderstandings if left unattended. Do not harass other users. There is a difference between criticism that is constructive and a pure insult; KNOW the difference. Insults are never okay.
An example of a contructive crit: The head on your figure is a touch too large for the body. You may want to watch for that in future images, as having good proportions will definitely improve your artwork.
An example of an insult: This looks awful.
The former is intended to instruct and offer helpful suggestions. The latter contains no useful information and can have no intention but to upset the artist.
You can ask for critique on any of the boards, but the Sage Advice - Critique Board is where you should post if you want the majority of the feedback you get to relate to constructive criticism of your image. If you post a comment in the Sage Advice board, please make sure that it is helpful and constructive.