Many people refer to it as a "TAG" when it's not, it is an attribute, and therefore is very important. In order to know what the "alt" attribute does, you have to realise that Google cannot see an image, all it reads is a mass of code that the website uses to display the images, Google and other search engines need something to tell them what the image is picture of, so that it can index it with similar images, and then show it when a search is made for that particular person place or thing. So if you thought the "alt" attribute was there just to tell users what the picture subject is when hovered, think again. This little attribute is must for SEO, if Google doesn't know what subject your picture is, then it can't index it. This is why you should always source your own images, if you can't then when running a forum particularly you should look for an add-on that adds this attribute for you. I know that there is such an add-on for VB4 and Xenforo, you will have to check for other platforms.