The short version:
use the following code in your post
<img src="http://upload.wikimedia.org/wikipedia/commons/6/6e/Napoleon_crop.jpg" />
The space at the end is VERY important. Of course replace the URL with the URL of your image.
The above code results in the following image.
If you start the img code on a new line, you will get the picture left aligned.
The long story
In this day and age, protecting ourselves online is a laborious process. That is why the following process is necessary.
- You must first post the image online somewhere, flickr or some other image hosting service (many are free).
- Then use the above XHTML code to insert your image
- Make sure your image is the appropriate size before using it, unecessarily large images will look horrible and create long download times
- Copy and paste will get you along way, just change the image URL
- You've only got 600 pixels to work with as far as width goes, so if you want your audience to see the entire picture, keep it that size or smaller.