Googler, Evan, posted a Google Groups thread discussing the various reasons why an image may not be found in Google Image Search.
Then, if your images are in the index, great. If not, then change the SafeSearch Filtering from the current filter to "Do not filter my search results" in the image search preferences. Then do the search again and see if any or more images are found. If your images are now found, then you know your images are classified as not being Safe (or being adult in nature). Yes, Google has been known to be very sensitive in this area.
Evan also adds that Google indexes and ranks images on more than just your alt text and title tags. Google also looks at the "ranking of the parent or linking page," the page linking to the image. Evan also recommends that you "consider how unique and compelling the content is beyond the image itself, with the context and information surrounding it." Finally, he says you must be patient.
Be very, very patient as changes do often take some time, meanwhile continue to provide relevant content for your users and things should eventually work out.
Forum discussion at Google Groups.