when you click reply, you will see this button on the screen if you have the advanced message controls turned on which I believe everyone should have?
this will pop up a dialog box, which might be blocked by your browser, so unblock CSS from the pop ups in your browser. Then insert the image url which you can get from the image itself by right clicking and then selecting properties.
My general feeling is not to hotlink too many images or none at all, since bandwidth costs money and it would not be fair for the site to pay for bandwidth which is not bringing in revenue, so either just post a link to the image or a link to the site. :)