It is up to you. It is basically a system that allows you to decide how you would like to allow your work to be used, while allowing you to retain whatever rights that you choose.
Unless there is a reason not to I would choose "Attribution-NonCommercial-ShareAlike Creative Commons". This would allow non commercial use of your work but you would be credited if it is used. If you don't want your work messed with pick "Attribution-NonCommercial-NoDerivs Creative Commons". The door is still, open for someone to offer you cash for whatever rights you wish to sign over.
I am a member of the deviantART community where where the opportunity to flag your work with creative commons attributes is provided. Because a lot of what happens on DA is reworking of stock images or resources, creating various types of mashups etc., "Attribution-NonCommercial-ShareAlike Creative Commons" is common.
flikr is a bit different because it is more or less straight photography with little reworking of others work.
It depends on the image too. You might have a strong sense of attachment to certain images, and not want them altered or used in a way that you don't approve of in advance. Other images you might not care so much about, or you might be interested to see what someone else may want to take off with it.
If someone wanted to take one of my images and smash capitalism (or smash the state or whatever) I would be OK with it. If a commercial venture were to make swacks of cash with it I think I would want to be able to say no way hosť or else take a cut (and use to proceeds to smash capitalism or smash the state or whatever).
[ 14 September 2008: Message edited by: scott ]