Add the Add-In to your Add-Ins folder.
If you're unsure where it is, in Excel goto-->Tools-->Add-Ins-->Browse to get the path. Once you've done that goto-->Tools-->Add-Ins and select HtmlMaker 2.42. You will then have an HTML menu. When you're ready goto HTML-->Select Used Range. The Add-In will automatically select a range for you, but you can expand or reduce it as necessary.
Follow the dialogue boxes and your spreadsheet image will open in a new window. There's a button below the image that says "Click to send image to clipboard". Click it.
Go to your post and type your message, then hit CTRL+V to paste the HTML. When you submit your post, your image will be there.
Hope that helps,
Smitty