Code:
<img src="{vb:raw article.previewimage}" width="{vb:math {vb:raw article.imagewidth} / 2}" height="{vb:math {vb:raw article.imageheight} / 2}" />
You can adjust the proportions as you need, just as long as they stay the same between the two options. And remember, you're not actually resizing the image, you're adjusting the size that the page is using to display it. By default, the CMS doesn't do a very good job with thumbnails...
As for your other issue, sorry. Tried it, but CSS isn't my strong suite.