Optimizing images for SEO is no secret, use the alt attribute on images and ensure that the image and the alt text are at least related to the topic of the page that improves your search rank while allowing you to integrate the necessary marketing messages.
Beyond that you should compress them and strip out any unneeded bytes (they’ll just slow down your load times) and name them something obvious, descriptive and related to the topic at hand.
Think about it like this: imagine you’re optimizing a travel site and you want to optimize a given page for the term “Las Vegas hotel”.
Let’s say you also want to include an enticing marketing message such as “Book now and save 20%!”.
The aforementioned tagline lacks descriptive text, but possesses persuasive characteristics. That being said, you may want to place the tagline in an image and the key phrase (i.e. Las Vegas hotel) in a header tag. This places emphasis on the desired term, yet still provides a marketing opportunity without compromising keyword consistency.
In other words, images are a great place to insert marketing messages that supplement and diversify the terms you’d like a page to rank for.
Leveraging this technique will ensure that descriptive text is indexed, while less marketing jargon is overlooked. The combination of keyword-rich content and enticing messaging will satisfy both sides of the strategic equation.