I get this question a lot, here are a few thoughts.
Think about it this way, the alt attribute might tell you what it is if it’s worded well. Bu, even if it is worded well, is it what the page is about? We don’t know. We can’t be sure because the alt is just a description of the the image. The h1 on the other hand is very clearly a very important heading. This is very clear.
Consider a page with these two images:
Is the page about cats or is it about pepsi? You can’t tell. The page might be about Pepsi with a picture of a cat enjoying the pepsi, imagine it’s part of Pepsi’s new ad campaign that takes advantage of the internet’s love of cats.
Or, in an alternate universe, imagine that the page is a cat fancier blog, with a picture their cat named Pepsi. It could happen, Pepsi is a cute name, they already have another cat named Moxi.
Now consider this markup.
h1 Pepsi /h1
img alt cat
The h1 tag clearly says this page is about Pepsi, whether it’s the cat or the drink might have to be determined by meta data. There is no mistaking that Pepsi is more important than cat. Pepsi is a very important heading, while cat is merely an alternate text description of the image.
Using this markup, we’re also clearly describing the content and presentation. Pepsi is the name of the site so we use text. Cat is a picture on the page, presumably a picture of a cat, the computer can’t be sure the picture is actually a Cat, or the logo of a multinational corporation. Consider that in the context of the first example for a moment. Catapillar tractor abbreviates their name as Cat. It’s printed on everything this. I seeing even more room for confusion in the first example.
Using this arrangement we also get the advantage of displaying the logo as text, or as any range of alternate images, based on media queries. The look of the h1 is fully handled by our stylesheet. Mulitnational corporations like to have their logos appear in very specific ways depending on context.
Overly clever readers are thinking, what about this:
img alt=”Cat, the Catapillar company logo”
img alt=”Pepsi our peppy new skid steer loader”
While this is more descriptive it requires a fairly intelligent system to examine the alt text and come up with a meaning. While the Google algorithms are pretty smart, I still feel there’s room for error. And all of the non Google algorithms fail.