If you want to insert a picture from your album into a post, you need to use your browser to copy the image's link. In Chrome on a PC, you would right-click your mouse on the image you want, then choose "Copy image address." Other browsers have similar functionality but might call it something a little different, like "copy image link."

Then you should "Go Advanced" to post a reply to someone or post a new thread from scratch. In the toolbar above the body of the post area where you write, you should see an icon that looks like a little image of a mountain. Click on that to insert an image. A box will pop up and that's where you paste (CTRL-V on a PC) your image link.

That inserts the code necessary to display your image in a post, like so: