When you take the screen shot, save it as a png file and upload it to an image hosting site, like ImageShack
. Copy the link to the file provided by the hosting site. When you're creating a message (don't use the quick reply when trying to post an image unless you know the syntax and want to type it out manually) click the Insert Image button between the Insert URL and Insert Email buttons. A window will appear asking you for the link to the image. Paste the link and click OK. When you submit your reply, you should see the image as you uploaded it in your reply.
If you let the install run all the way through after it says it's already installed, what happens?