If you run Site Auditor on your WordPress blog or website, you may discover that many more of your images are missing title tags than you might expect — especially if you've diligently added title information to each image as you uploaded them into WordPress's Media Library.
The reason for this is that WordPress, as of version 3.5, doesn't pass Title information into embedded images. Instead they use this detail internally, to help with searching and sorting your uploaded images in the Media Library itself. To restore that functionality, you may want to consider installing a plugin like Restore Image Title.
Another thing to keep in mind that not all of your images may be managed in Media Library. In particular, images that have been hardcoded into your WordPress theme can be missing this information. If updating the Title field in your images and applying those parameters via a plugin don't solve all of your title tag issues, be sure to check your template files to ensure that all of your theme's images have this information.