Hi there's an explanation of what you can do on this page. Since you have all your butterflies set up as buttons, you can use the Buttons and Forms palette to name each of your buttons and that will add alt tags. I tried your app with VoiceOver on my phone; I think that would help a lot as well as alt text for the pop up images using the Object Export Options dialog box. I noticed that only the title of each pop up was read by VoiceOver--you might want to look at that as well.
Hope this helps!
The link Maurice added has the best advice for how to add alt text.
It sounds like a lot of content, so you might want to check out this article for advice on long documents: Tips & Best Practices for Exporting Long-form Digital Documents from InDesign with in5 (and speeding up load time)