Added alt text to img embeds

Since it's good practice to include those for people browsing while the image host is down, or the poor souls using Lynx. 

There's no good way to programmatically decide what should go there, or to have the user pick one, I just went with the image url itself. So, if someone sees that they know exactly what they're missing. Or at least where it is.

But really I did this because the validator kept bugging me.