You can post art as a regular forum post or you can draw something on the Oekaki drawing program and post it that way.
If you want to post your art, you can created a topic in this forum:
Forum/viewforum.php?f=10Upon doing so, you can upload the image you want to somewhere like Imgur or DeviantArt and use a link to display or share it. Or you can add the file directly to the post. To do so, look below the text box when making a new post and look for the tab that says "Upload Attachment." There should be a button that lets you upload the file you want. Be sure to click to add the file when you're done!
As for art trades, they are allowed, but take a little searching to do!
Some people try to find art trades here:
Forum/viewforum.php?f=21(This is the art sales, but art trade topics seem to be allowed)
and
Forum/viewtopic.php?f=10&t=3000397Just be sure to state what you're looking for and give examples of your own work. Generally people will tell you if they're interested and then you can work out the specifics.
You can also make an Oekaki post seeking art trades, but be sure not to just write on the canvas, it's meant for drawing, not writing, but if you include some art in the piece to show your examples, that is fine!
As a third option, you can post in your signature that you're looking for art trades and ask that people PM you for details.
If you need help with anything else, just ask!