How do you post pictures in a post that show up full size in the post and not a thumbnail that you click on to see full size?
IIRC, upload the attachment, (manage attachments, the paperclip icon). Once it's uploaded, click the insert image icon (a yellowish, postcardish looking icon) and paste in the link you get from the image upload. That shows both the image in the post and the thumbnail at the end of the post. If that's not exactly the right steps, it's close to the right steps.
If you click the down arrow to the right of the paperclip and choose an uploaded file, it shows the thumbnail in your post, that will expand when someone clicks on the thumbnail.