Good question. To post an image you have to host it somewhere, then use an html tag to bring the image into your reader review.
Before you get to posting pictures all over our site, though, there are some things you should consider: Size, Suitability, and the Severity of your punishment should you post anything inappropriate.
Let's say you're reviewing Half-Life 2, and trying to post a gigantic image of your grandmother lighting herself on fire somewhere within the text. If the picture is gigantic, it's too big. You should resize it down to 300 by 225 at the largest, and make it a jpg image of no more than 25 kb. That way you won't clutter up our site or stall people with slow connections.
Unless your grandmother and flames have something to do with a point you're making about Half-Life 2's graphics or gameplay, please don't put her on our site. This is *not* my space - personal or random images are not suitable for reader reviews.
Still, posting your grandmother's last moments isn't something we'll get super severe about. There are things, like naked people, orgies, bukake, animal cruelty, fisting, dog fart, bad words and links to sites dealing with all of the above and worse, that will make us really butt hurt if you put them in your reader review. You will never be welcome again, and we'll stop allowing people to post images.