You want to look at this from what you think your user will be search for. There is a lot of traffic you can obtain through image searches. So if your title and alt tab reflect the search queries, then your images will show high on an image search.

Also, consider pinning your images in Pinterest. I have gotten a lot of traffic through Pinterest for our images, as well as some SEO link value from Google.

Need more information to give an adequate answer. There are so many factors that can influence it on both sides of the fence. (Google's side and your side) If you have not changed anything on your site and have not done any off-page SEO, then you are seeing the changes with the algorithm that is affecting your ranking. Google is making changes almost every day, and it will affect your rankings, and those of your competitors if those changes are targeting certain attributes on your pages and other's web pages. As you add more content and pages to your website, it tends to settle down on movement.