The simple way is with that handy img button at the top of your post. Click it, and you'll get text in your post that involves "IMAGE HTTP ADDRESS GOES HERE" Replace those CAPLOCKED WORDS with the http address of the image. If it's an image you have a copy of on your home computer, you can easily create a free account at a place like www.flickr.com and post it there, and thus get the URL from them. If it's one you don't have a copy of yourself, find the image in a web browser. Right click on it and select the option along the lines of "view source image", "view background image" (depending on which browser you're using). That should give you a browser with just the image displayed, and the URL for it should end in an image file--something that's .gif or .jpg or the like.
For example: if something works right, we'll say it's a miracle. So let's post a picture of an angel. But, we're gay men, so let's make it a hot male angel.http://farm3.static.flickr.com/2164/1696618585_3e9b67fb39.jpg
seems to fit the bill.
He's on someone else's flickr site, so following the instructions embeds him like so:
That's all there is to it.