1. Yes html is fine but keep it simple and test it thoroughly. A readme is no good if somebody can't read it. And keep it small: pics are tempting but can rapidly make for an unneccessarily big file.
2. Clarity, simplicity and relevance.
3. To make a good Overview first of all you need to get the technicalities right: a decent picture (good composition and so on) with a good border, and the right amount of relevant text. However, a really good Overview is one that grabs you: makes you think "Wow! I want to play this."
Once upon a time a famous short story writer came to speak to an English class. "The things that make a good short story", he said, "are brevity; mystery; religion; aristocracy and sex". After he had gone, the teacher set the obvious task - write a good short story. 30 seconds later somebody put up their hand. "I've finished!" he cried. The teacher was outraged - you can't possibly write a good short story so quickly - and ordered the student to read the story to the class.
"Oh God!" said the Duchess, "I'm pregnant. I wonder whodunnit?"
Writing a good Overview is much harder than this of course, because not only must it be a good short story on its own merits, but it must also urgently convey the message that the mission is an even better story.
And in a campaign, naturally you have to do all this with a picture alone. That is one of the reasons that mission making - good mission making - is hard.