Just a guess here, but I think the fact that there are more and worse Iraqi prison photos yet to be released has played a role in Rumsfeld’s failure to resign. If he resigns now, people will demand still more action at a later point when those other photos and videos get released. Better then (the Bush administration thinks) to wait until the other evidence is released and at that point have Rumsfeld take the fall, if necessary.
Different issue: some American civilian contractors involved in the abuses are considered above the law because they’re arguably not subject to US military law, but are also not subject to Iraqi courts pursuant to an “agreement” made by the US occupation. Seems like there’s an easy solution to this – the US waives the particular civilians’ immunity to Iraqi prosecution. This should be one of the demands on the table for ways start cleaning up the mess.