I can't get "middle" and "corner" into the same sentence. Do you mean "edge"?
Cut & paste from my experimental picture:
position: fixed; right: 10px; top: 50%;
"right: 10px" means the right edge is always 10px from the right edge of the viewport. (If it were "position: absolute" it would be 10px from the right edge of the page, or potentially out of sight if the page won't fit horizontally.) So "right: 0" puts you smack against the right edge, while "left: 0" would zoom you over to the other side. If you've also set a width, as you would for an image, the other side should default to "auto".
"top: 50%" means that the top edge of the picture is halfway down the viewport. If you want it centered you have to use a percentage. And if you want it perfectly centered you have to make it a background image; only backgrounds have an option for "center". Otherwise, take your best guess at a reasonable percentage value, like 40% or 49% or whatever suits the size of your picture.
There will be a lot more options in CSS3, but the "position" section is still very preliminary. As in: red label rather than blue or green. But scroll through any CSS3 informational page and you can see fixed positioning
[dev.w3.org] in action ;) OK, technically it's a background (I sneaked a look at the CSS) but the effect is the same.