Thumbnails when window is minimezed in DocbarX

Well, minimized windows technically don't have a preview since a minimized window is destroyed, in other words there are nothing to preview. There are ways around this, ugly hacks most of the time. Windows 7 takes a screenshot of a window before it gets minimized and uses that for the preview. KDE doesn't really minimize the window at all but moves it outside the screen.

DockbarX doesn't take care of delivering the previews itself (it did in x.0.30 and x.0.39 but it didn't do a good job). (Since version x.0.39.1) DockbarX lets the window manager (Compiz Fusion or KDE's kwin) take care of drawing the preview. All DockbarX does is telling the window manager what window that should be drawn and how big and where it should be. This is the only sane solution for a project like DockbarX, things like drawing previews should be done by the window manager, the dockbar team don't have the resources to do the work for them.

If you want previews on minimized windows you have two options:
1) Use a window manager that supports it: kwin. It's KDE's window manager but it can be used in gnome as well (I haven't tried it myself though).
2) Demand that your window manager (Compiz Fusion) adds the feature. The compiz team has said that they are not planning on making a hack for feature. They are instead waiting on real fix for the problem from xorg. Every one else seems to implement their own hacks (kwin and gnome shell for example) so the compiz team could probably be buggered into changing their minds.

Matias Särs suggests this article as an answer to your question:
FAQ #1181: “Why no thumbnails when window is minimized in DockbarX”.

I know this old question but I enabled the workarounds plugin in compiz and enabled keep previews of minimised windows. Also enable the KDE Compatability plugin under the gemeral tab of the KDE Compatability plugin enable support for plasma thumbnails and enable support for present windows fixed the issue for me.