1) No spam.
2) No foul language.
3) No derogatory comments towards any person.
If I think a comment is against the above rules, I will remove it.
1) Edit: After commenting you can see the date of your comment to the right of your name. If you click that date you will go to the corresponding discussions page and there you can edit your comment.
2) Images: You can include images using the URL of the image. You can also upload images to GitHub if you comment on the corresponding discussions page. So you can first write some comment and then edit it on the corresponding discussions page and this time you can directly attach images.