Yes, it's one of them.
<title> attribute provides "advisory information", and is not indexed by search engines. See this for more on that: W3C - The title attribute
In addition to the title attribute, you should be using the alt attribute, as covered here: Matt Cutts Discusses the Importance of alt Tags
Google AdSense needs to be able to find content related to keywords. With a site solely made up of images and no other content, it's unlikely that you'll be approved. You should add text that can be read by visitors without hovering over images (as is the case with the above) so that AdSense can display ads related to the search terms that led to your site and the content contained on it.
In short, you should add HTML content, in addition to using
alt text for images instead of just having title text.