Google never says exactly how they rank your site or what will be part of the search criteria. But every part of the page, including the alt tags is fair game.
Alt tags exist to describe the images on your page to people and devices (like search engines) that can’t see and interpret the meaning of the images.
Every image in the HTML of your page is REQUIRED to have some alt-tag to make the page valid HTML – even if the images are used for mundane purposes like bullet marks of a list or a background gradient. For this reason, and because it’s a requirement for valid code – Stacks adds automatically generated Alt Tags to every image. This is sufficient for validation – but not great for Google ranking and indexing.
To get great rank and have your image content contribute to correct indexing of the page you should give at least your prominent images an appropriate Alt Tag that describes what’s in the image or why it’s on your page. Google will pick those up and improve your rank and index your page with that new content.
But Google indexing can be SLOW – sometimes as long as 2 weeks to pick up all changes. If you dig deep into some of the Google tools there are ways to encourage Google to re-index your page. But be patient – I’ve seen many many cases where Google only indexed part of the site after a couple days – but the deeper pages didn’t get indexed for weeks.
In general, the more traffic you get from Google, the faster it will index your pages. But there are no guarantees. Google’s methods and black magic alchemy and they don’t let any of their secrets out at all.
Google is generally quite good at determining which images are prominent and important to the viewer – so you can often cut a lot of busy work and leave some of the background gradients and other fluff images with their automatic filler names. Or, especially if your business relies upon your Google search results, you can be 100% thorough and name every image on every page.
Partials can come in handy here. If you have an image that is used the same way on many pages then make it a partial and add the partial to those pages – then when adding or changing the Alt Tag to it will affect that image on every page.
Alt tags in Stacks can be editing by double clicking an image and then adding an alt tag. Here’s a quick image to show you where: