Its best at the start to broadly think about how you want to present the material -- but don't worry you can mix things up and shift it around as you go. Very very roughly, there are 4 classic styles:
a) gameplay - in effect you report what happens in the game, this can be of the style "1405 - the Swiss invaded Sweden", or "thinking about it I decided to move my National Focus to Lothian", in other words you're either explaining what is happening in game or your logic for your moves;
b) history book - not always that different, but you'd tend to write it as "In 1504, King Charles called a council of his nobles. After many days, they agreed to his plans. The invasion of Russia was to be the next step". So you're sort of converting (a) into a fiction
c) narrative - wouldn't do this for your first go. In effect you base a totally fictional tale on the game you're playing. It can be a study of an unimportant soldier in an army ... or anything.
d) humour - which can sit across any of the above but often (a) as the base. This is actually hard to write consistently well, but a lot of good AAR writers (Naggy is one) add a lot of humour to their posts as they go along.
Images need to be hosted somewhere - I like image shack, especially if you download the free uploader. You then embed links to that site in your report.
Best advice, wander over to the AAR forum, especially the generic page ... lots of good advice and lots of people will help you out. There's a thread by Slan precisely covering how to embed and upload images.
Finally, don't worry if at the start you get few readers or comments, just keep writing and people will start reading. In effect, write for your own enjoyment - its also a good way to improve your own game play as you have to stop and analyse a lot more than if you are just playing.