Don't forget Title attribute and Caption
I have had good success getting images noticed by Google by including the title attribute in any link using the image: title="put title here", goes in the opening anchor tag. Also make sure you use a caption with your keywords for the picture. Google looks at image file name, alt tag, caption, title tag in any link associated with the image, and text on the page near the image.
And one more thing, Google also looks for fresh content in its image search. If you want to make sure your image stays on the first page of image results you need to keep that image fresh by reusing it every couple of months or so in a new post/page.
You can also set up a special image sitemap for Google that will help them index the images on your site.