2 ways :
If all you need is an expandable thumbnail, then use the "Manage Attachments" down below the text area of the post. You click the 'choose file', pick the file(s), then do upload.
When upload is done, click 'close this window ', then 'Submit Reply' for the post.

Second way :
Load all the pictures into one of your albums, then you CAN copy and paste to the post.

album example :



thumbnail example :