The Alt Tag provides very marginal if any boost in SEO. It certainly is a bestpractice to use Alt tags, but, I wouldn’t rely on them to provide a noticable SEO value.
The most important Alt tag is on the logo. So, you might be wondering, “How does Google know it’s your logo?”
Should it be called out in the filename? Well, there’s no Google “Logo Search”…at least not yet. Hmm, a vertical search engine that looks at logos. Not a bad idea.
Certainly, a logo should be tagged as a logo in the alt text for accessibility purposes. But, Google can’t differentiate the logo from other images. So therefore, perhaps the bestpractice would be to alt tag it – “logo”. Google can only know from the alt tag or file name. I wonder if tagging it “logo” would provide the extra wording in the Alt text any more SEO value…like, “Logo – SEO Company”. Although, I doubt it; sounds very 1999.
In my opinion, the most important Alt tags are on sitewide images.
Think about it. It’s just logical. Google must place more importance on a sitewide image over a single page image. So, sitewide images get more weight, not just logos. All things being equal between 2 sites, the Alt tags properly written could push 1 site over the top.
So, as far as Google being able to differentiate your logo from the rest of your images, they can’t. Google cannot OCR (optical character recognition) the whole web – thats why even links labeled sponsored links, Google can’t spot. The web is just too big for them to crawl and read all the graphics. Even though the bots can read the text in images. Thats why captchas don’t offer full-proof protection these days against spammers. Because the bots that OCR them are getting better.