If you have the rich text editor turned on (i.e. you can see the little bold, italic buttons etc when you write a comment), you can't. You'll have to post a link to the picture using the link button.
If you have the rich text editor turned off, you can use the normal HTML code to add an image. For those of you who aren't completely HTML savvy, the code is
To turn off the rich text editor, go to your account settings and untick the box there.

lol just testing...
^ARGH. I have the stuff typed there but it's still not showing.
I used one of those Kelly Clarkson pictures on this site. I have it typed in as follows [no gaps]:
Okay, now I'm wondering why it worked on the bottom one and not the top one. Nevermind, I think I have it now, thanks again.
THANK YOU, I SHALL HAVE ANOTHER GO!
I took off the rich text editor and I type in
url with no spaces and it never shows. What am I doing wrong please?
< img src = "image url.extension" >
Like that, without the spaces...