To the left of the option "Suggest a fav" is something that says "Contribute Art".
Click on that, Click on the art you want to upload here, Select a category, and click "Submit" Then it shall be reviewed/Accepted.
Also, Certain groups don't let you post unless you actually joined. How to do that is go to the right of the thing that says "Suggest a fav", Which the thing to the right of it should say "Join are group". Click on that, And then you can type reasons you should join. Then just click Submit join request. You can also submit your join request without even typing reasons you should join.