When you are in the Post Editor after hitting the "Post Reply" button there are these controls above the Text Box. (Sometimes they are next to the Text Box when using a mobile device.)
Clicking the "Img" button will insert code that looks similar to this: [img.] [/img]
Paste an image's address (your image must be uploaded somewhere online, like Flickr or Photobucket) between those codes so that it looks like this:
[img.]http://photo.c0m/fake[/img] Voila! You're done.
Otherwise you could use the "Imgwidth=" button to control how large the initial display size is. Same technique, but with this one you enter a specific size of your choice, in pixels, after the equal sign.
Example:
[img.width=500]http://photo.c0m/fake[/imgwidth]
I hope that this helps!