Depends.
If the pictures are on another site usually you can just copy the link to the picture and paste it into the post using the little picture icon that puts IMG tags around it.
Otherwise, you can save it to your photo album here, then copy and paste the picture link same as above.
Or, you can attach the image to the post, click the thumbnail, when it opens in a new window copy that link. Then, as above, post it into the thread using the picture icon.