Using the Tiny MCE, select the first icon on the left in the bottom row. This is for inserting images into a post. If you have already uploaded the image to our PSA media library, copy and paste the file’s URL (not the permalink) into the URL box in the insert window. voila.
I’m doing this one for you, but as a tip: if you forget how to do something, you can find a post where it is done, click on edit the post, and then examine what’s there in front of you and you may recall how it is done by looking at it, or editing it some more.