Slick trick to insert full-sized image of attachment in your post:
1. When viewing or previewing post click on attachment to display image.
2. Copy URL of image (how to do that depends on your OS)
3. Paste URL into text of post bracketed by [IMG]ImageURL[/IMG] making sure that there are no spaces between two tags.
(Alternately you can click on Picture icon and paste image URL into dialog box.)
4. After saving edits you now should have full-sized image in your post.