For a long time I suffered from blurred icons in the alt tab screen of Openbox. Until I realized that for some reason it uses non-standart resizing.
http://icculus.org/pipermail/openbox/2008-December/005964.html
For the same unknown reason the developers haven't fix this. So if you want sharp images you have to fix it yourself in the source code:
grep config_theme_window_list_icon_size openbox/config.c
After that you manage to get something like this: