First things first. Yes, you can EDIT your comments. We live in a bold new world.
Just click that little gray gear icon to the left of your comment, and a menu will pop up. At the far right, you can see the "edit" option. Click that and you can edit out that unfortunate spelling error from the comment you just posted. Sorry, you can't delete comments. But you can erase the comment and replace with the word "deleted."
You can also post images and video. To do that, go into a comment field and click the left icon to upload an image - or click the YouTube icon to post a vid (it will ask for the YouTube URL, and then will automagically grab it). When you've finished, click "share" and your image or vid should show up.