iOS web dev: orientations & choosing image sizes

I'm building an iphone version of a website and was curious of the best practices for choosing image sizes. I'm making a simple vertical column of images text. I originally thought that 300px would be the best size (320px portrait mode minus 10px margin each side). However when user rotates to landscape the images will now seem blurry & upscaled. The alternative is to make the images larger, but then in portrait mode they will all need downscaling- does this make the page load slower? I assumed most people view in portrait.

If I go with landscape/4开发者_开发百科80px as the base size what viewport meta should I use to get it to size down for portrait? My current viewport meta tag is :

<meta name="viewport" content="user-scalable=no, width=device-width" />

Many thanks in advance