Hello!

To place an image, you'll need a direct link to your image - I personally use imgur to host my images.
Then, you need to place the direct image link into the following form, without the * symbols:

<*img src="LINKhere"*>

As for the other question you had - usually, you draw your adoptables, make sure they are not breaching our rules (aka. you made the art 100% by yourself etc.) and place them on a thread, saying how much would you like for them. People bid/buy your adoptables, then you send them the link to their adoptable image to make sure they have it grabbed, then you can remove the purchased ones from your thread.

Hope this helps!!