I would tend to agree with William here. The objective of the alt tag is to help give context to the image if the image cannot be displayed. If you are able to describe the image in a way that relates to your post, you will be better off than trying to stuff keywords in the alt tag. If an image is related to the post, search engines (particularly google) should gain a contextual understanding which is better in the long run.
In my experience, I find that the more I try to "optimize images", the more unnatural they are. Again, we are trying to be natural and helpful.
Hope that helps.