It likely indexed that one from the blog homepage, as you have the photos there also.
As to why Google is ignoring the alt text, it may have indexed it before you added the alt text, or the algo just decided differently.
There isn’t much more you can do to control those results, but if you’re into custom code work there are a few things you can try;
Add image title attributes as well
Possibly JSON-LD to highlight relevant images
And then the next level would require a reverse proxy-
I’m not sure which solution approaches you’re considering.
Local-code solutions like adding titles to your images is relatively easy depending on how you do it. You could just find all images, and duplicate the content of the alt attribute, depending on how your pages are setup and what text you’re wanting to attach to each image.
Google or chatgpt will help you find those solutions and craft them to your specific HTML DOM.
Reverse proxy solutions are more complex.
I build these often for clients, and it’s often for SEO reasons - but in my view in this situation that’s probably overkill. I’ve not seen evidence that e.g. images in the sitemap makes a big difference to your SEO and it would be quite complex to build that performantly,
Search the forum here if you’re interested to learn more, you’ll find discussions and tutorials.
If you’re looking for a developer to help you build a solution, drop me a line- but first I’d suggest you research the SEO impacts of whatever feature you’re wanting to add to make certain it’s worth the effort. Most of my focus so far on reverse proxy SEO work surrounds path flexibility, semantic paths and keywording.
