We're sometimes asked about optimizing photos (images) on websites, and more speficically, the difference between alt text and image titles.
The guiding principle here is that the text you enter for either one of these should be useful to visitors. Keep in mind that alt text will be used by visitors with disabilities where, instead of viewing images, their computer "speaks" text to them. Other visitors at your site without disabilites may also be browsing without images enabled, and this information will help them consume your content more effectively.