You can embed the whole viz using the embed code: I have done that on my wordpress run blog: datadrivenconsulting.com/blog/
I used to have to edit the code slightly to remove some whitespace, but it did work.
There are a couple ways to grab the image. If you're looking at the preview window of your viz, you can click on the box at the bottom that has an arrow icon on top of it, and choose to export an image. You can also type in this on your web browser:
http://public.tableausoftware.com/views/[workbook name]/[dashboard name] and add ".png" to the end (instead of the usual ?:embed=y). That will net you a PNG file too.
As Alex said, you can also embed the entire viz using the embed code (click 'Share' in the lower-left corner of the viz'). If you're using a Wordpress.com site (hosted by Wordpress) they may block using the embed. If you're using a Wordpress.org system, you should be able to embed the viz without much trouble.