For your images to show up in Google Image Search, you don't just need to make sure your images are on the web somewhere, you need to make sure that the image itself is on a web page that can also be indexed. If either the image itself or the associated page the image is viewable on is not able to be crawled or indexed, then Google won't show it in image search.
John Mueller from Google offered this advice once again on Christmas day in a Google Webmaster Help thread. John was responding to a complaint about an image in Google Image Search. He said:
I suspect there's something not quite working as expected with your web page removal. In order for an image to be shown in Google Images, we need both the image and an associated landing page indexed. If the landing page has been removed (or temporarily hidden with the removal tool), then we wouldn't be able to show the image either. My suspicion is that there are multiple landing page URLs involved (sometimes this can be hard to spot, with subtle differences in the URLs of the pages), and one of them has been removed, but the others are still indexed. If that's the case, work to get the other landing pages removed as well, then this should result in the images also not being shown (you could also take it further and also remove the image URLs, if this is on your own website). Sometimes this takes a bit of practice, but it sounds like you've already been able to get one version removed, so you probably "just" need to focus on the exact URLs involved, and clean those up too.
Forum discussion at Google Webmaster Help.
