We had a request a couple of years ago to replace "enter image decription here" with some sort of generic text. I think this is a bad idea, as it doesn't add anything. Shog's comment on the proposal essentially articulates how I feel:
I would rather it changed to something like, "Author hates blind people".
So our only other option is to enter useful descriptions by hand.
This Google search shows which posts are still in need of help.
I will make a list of posts that still require fixing, and we can remove them from the list as we go. Feel free to add any that you find yourself.
You don't have to write like Shakespeare or Nabokov, just something that makes it clear what the picture is of.
Obviously this is a big task, but each picture adequately described is a little victory!
If you're interested in tips on how best to fill out an alt text, this site has some ideas.