When I hover over an image, the preview would be displayed in the viewer pane.
How do I manually clear out the preview?
This would only happen if you had the 'single-click' mode enabled, where hovering over a file is equivalent to selecting it. When you select a file in Opus the preview pane shows it, as it is designed to do. So in short, you can't 
Is it possible to at least not show any preview when I exit out of that folder?
I too find this annoying so I have disabled the preview pane just because of this. In windows explorer when you click on a picture it'll show a preview and when the focus is out it'll clear the preview automatically. In DO you have to click some other file for the preview to clear!
DO should auto refresh it's preview. There's tons of options available so why not add this?
...and I'm talking about selecting a file, not hovering.
Just toggle the preview pane off and on again if you want to clear it.
It doesn't seem that useful to me for it to go completely blank if you select a folder or something. What's wrong with keeping the previous image displayed until something else that can be viewed is selected?
Maybe NSFW material? 
Personally, I don't care one way or the other, but I suppose it is a little disconcerting having an image (or whatever is being "viewed" via a plug-in) linger there after one has moved on, so to speak.
As a follow-up, I guess the "disconcerting" thing is that DOpus breaks the model used by all other applications that have preview-type windows, e.g., ACDSee, Media Center, Photoshop, etc.
The model is that a preview should only be shown for selected items, and once that item loses focus, a "preview" is no longer shown.
Again, though, it personally doesn't bother me, but I can understand where the OP is coming from I guess.
I agree with Zach, but I also think that it's unwanted from a usability standpoint.
What happened to me today for example, was that I went to preview a picture and then switched folders (preview still visible). Then alt-tabbed to other programs to do other work, and after a while I tabbed back to DOpus. Since the preview is kind of the first thing you see, my mind instantly assumed that DOpus was opened in the picture folder where that picture was located. However, when I wanted to select another picture from that folder, it wasn't there in the listing! My mind had to reorientate for a few seconds (checking tab name, checking listings) to realize that the preview was still visible from a folder I had visited quite a long time ago, and that the folder listing was showing a different location than I had expected..
I suppose the model used by other applications not only makes you get used to that behaviour, but also has some sense in it
That's exactly how windows explorer functions and that's how it should be.
Given that Opus has had image previewing for 6 years and Explorer has had it for 6 months, why does Explorer get to set the standard?
Have a play with the Vista preview pane some time and decide if you really want Opus to be like it...
A "standard" can become different than the first implementation for good reasons, but setting that aside, what about the usability point of view? (as I sketched above with an example)
i would like to contribute that i actually like the way opus handles it's previews. i often use the opus viewer pane to allow me to quick play an mp3. i like that opus will allow that which is currently shown in the viewer pane to remain there until another file is selected, irregardless of to what folder one changes. this allows me to have a song playing in preview or have a document i'm reading currently in preview while i move through other folders.
mebbe an option to allow different behavior for different filetypes could be an idea...
wow, i had forgotten windows has viewer pane until this thread...
Yes, an option to selectively control the behavior of the preview pane would be great.
I agree with the several perspectives in this thread. Keeping certain files locked in the preview pane like how mluna states it is something I never thought of before and is pretty useful.
I have been using xplorer2 for over a year now and have become accustomed to how its preview pane works. It automatically previews whatever I have selected. If I have a folder selected, a mini thumbnail of the images in the folder is displayed in the preview pane. It dynamically switches regardless of where I am.
If it's important to you that an mp3 keeps playing longer than you select it to preview it, why don't you add it to the queue in a simple, quick loading mp3 player like foobar? (or double-click to play immediately like in preview)
I also wonder, if you browse elsewhere and it's still playing, how you remember where the mp3 was located that is currently playing. If you decide you need/want it, how do you easily get back to it without searching? (back button doesn't help) Nothing in the interface tells you the location of the current item in preview.. and isn't the preview there to be related to the current selection? (everything else in the interface is related)
Same goes for a document, which could just be opened if you find it important that it stays open. A preview is just that, and not meant for prolonged display imho.. If it's really important that it stays open within DOpus, you could always open an extra tab, right?