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.
An image title, on the other hand, is less of a description, and more of a...well, a title. As conventional wisdom would tell you, keep your image titles short, descriptive, and impactful. If your image is a chart illustrating advertising costs compared with new business leads, an appropriate image title might be "Advertising Cost vs. New Leads." It's simple, short, and accurately depicts what the image is about.
Optimize For Humans First, Then the Search Engines
In both cases, consider your visitors first, and optimize your images for everyone. Where it's natural to do so, also include keywords that are relevant to that image.
As always, remember that keyword stuffing is keyword stuffing, no matter where it happens. Use these areas of your site to help humans first, and then search engines.