Originally Posted by beyond11
Adding ALT tags in you images is necessary step to improve your website's SEO ranking.
No, it isn't. Really.
Over the years, search engines have been inconsistent in their treatment of alt attribute text. Over the years, Google has at various times indexed NO alt attribute text, indexed ALL alt attribute text, or indexed only alt attribute text associated with images that were links. Regardless of whether the text is indexed, there remains disagreement over how significant an impact it has.
In my experience, there are a lot of other things you need to have gotten right with SEO before image alt attribute text is going to make any sort of visible difference (if it ever does).
Use alt attribute text as it was intended to be used -- to assist those who cannot (or choose not) to view images on your site. The alt attribute text should provide a text alternative to the image, not a "keyword-rich" blurb written "for SEO purposes."
Stuffing keywords in the alt attribute text when those keywords do not describe the image itself could, in fact, get you in trouble with the search engines as evidence of "over optimization."
(And, of course, because you can't count on the search engines to consistently index alt attribute text, you should never include valuable information ONLY in images. If it's important for your SEO, it should be included in the on-page text.)
Originally Posted by Piers Rollins
Usually, search Engine bots/robots won't read images, PDF and Videos.
Uhm... this information is slightly outdated. Search engines have been able to read PDF files just fine for several years now. As a matter of fact, I often find PDF files ranking in the top 10 for various queries.