The suggestions from other members on the basics of optimizing actual images are good but those will only get you so far. Because your site doesn’t have a lot of text content on it, you will have to focus your efforts more on off-site SEO. I would focus on building backlinks and being active on social media. These are some of the most powerful things you can do for a site besides the onsite optimization. You can also do other things related to SEO for on-site that aren’t related to text such as making sure the site is fast, making sure it has relevant titles and meta descriptions on its pages, and things like that.
Here is a pretty good technical audit checklist and you could sift through it and do the things that do not involve text (such as the content section).
Hope that helps,