Version: (using KDE 4.4.3) Installed from: Debian testing/unstable Packages Disclaimer: Maybe I just did not find the proper way. Sorry if that is the case. While you can enable file previews for specific directories via View -> Preview, there does not seem to be a way to enable this feature by default. The obvious place for an option "always show previews" seems to be: Settings -> Configure -> View Modes -> Icons
PS: The same is obviously true for Konqueror, but I assume it's the same KPart, anyway.
Change that in the settings: General -> Behaviour -> Use common view properties for all folders