cliffsvoice - "How many users of WP are blind?"
it's not "users of WP" you are concerned about - it's visitors to your website, on the internet, that use screen readers (software that verbalizes the content on the site so they can hear it instead of read it, considering they're blind)
the "mobile" thing is for people who access the internet though things like cellphones and PDAs.
using "alt" tags are so that when people are using screen readers, cell phones, and PDAs - they can still know what the image is about, even though they can't see it (usually people who access the internet through cellphones or PDAs will turn off images, and of course, we're back to the blind as well on this)
all you do with an alt tag is insert it in your image placeholder tag, like such:
<img src="source/ofyourimage.jpg" alt="Description of the image." />
fixing that up appears that it will solve about 28 of your 32 validation problems.
now, all that said - i believe in validating, because it helps when your site "breaks", and aids in browser compatibility issues, and yes, aids those with screen readers or cellphones and PDAs. but there are a lot of people who don't believe in it. the choice IS yours, however if you're building your site to reach other people, then it'd be safe to say you DO "really have to". if you're building it for yourself - then who cares? but you're not - so you kinda do need to fix these things up.