Suppose I wanted to post this image in a Daily Kos diary?
![_3012407-2011-09-12](http://farm7.static.flickr.com/6179/6171547036_dfecbf026d.jpg)
There are multiple tools you can use for this, but Flickr is common and easy to use.
I did a quick screen capture to illustrate how to upload it to flickr and then copy and paste the HTML code you need. I recommend watching it in fullscreen mode.