If you are creating your website from a scratch, try to have only few images, use CSS instead and if you need to have images, allways use tags to tell spiders what picture is about, because they can't read images, so they need some kind of explanation, this what image alts are for.
Don't try to put to much keywords in image alts , this is bad try to keep keyword density in image alts on one page between 2-6% at max.