You can upload a photo then insert it inline, making it full size (or restricted full size based on the restrictions set by the site.
Select the advanced editor
Upload via manage attachments (make sure it conforms to the size restrictions of the site)
You should see the image attached under "attach files"
right click the image file linked under "attach files" and select copy link location or copy image link, based on your browser
Select the image icon in the editor. Looks like this
and paste your copied image location into the box
You should see it displayed as I have it here (note you will see both the smaller images "attached thumbnails" as well as the full size image (see below) ** update you may or may not see the small attached thumbnails **