one question - how do you manage to attach a file not at thumbnail size, but just the original size?
You don't "attach" it. You put a link to it in your post, using an [img] tag, I think. You can can use the leftmost of the second row of icons in the composition page.
FWIW, I don't like the habit. It makes pages slow to load and (at least in Safari), if you're at the bottom of the page, the text jumps about a lot as the images above load. If you attach a thumbnail, I can decide whether or not I want to load the large image. The first time, I probably will; later on, particularly in threads such as this one, I probably won't.