Taking your questions in reverse order:
Frankly, you are better off turning off the preview pane entirely. Otherwise malicious e-mails will be opened as son as you click on them: Click View/Reading pane/Off.
Regarding showing images:
Click the menu Button
Options/Safety Options
Select the Security tab
In the Download Images section, you can uncheck "Block Images...", but once again, I wouldn't recommend it. If you check both boxes, when you double-click and e-mail and open it, you'll see a yellow bar at the top (if the email contains links). You'll have the option to add the sender to your Safe Senders, list (in which case images from this sender won't be blocked again), or Show Images (in which case you'll see the images, but your answer won't affect any future emails from this sender).
IMNSHO these settings are the best compromise between safety and convenience.