How to get picked up by Google Image Search

So its time for me to ask a question, how does Google and Yahoo handle decisions in terms of listing images and which site to trust. It seems to me that the originating site should get the credit for the images, but this is not true in all cases. I would love to hear feedback of experiences on how to get images in the index. Simply turning that little switch in Google Webmaster Tools seems to do little or nothing to really help.

Should the images be hosted on the same domain? Does that make any difference?

Should the images be a specific type? Seems to me that this should not be the case.

Should the images be free of any java script?

Should they have a enlarge feature?

Obviously and ALT tag can help and a decent file name, but this does not seem to make a difference in certain situations.

Would love to hear opinions on this.