A lot depends on what device (or devices) your app will run on. This is also a big reason why Android comes with the different /res subdirectories, so you could supply different resolution images for different devices that have different screen densities and sizes. This page from the Android Dev Guide does a good job explaining it:
So one answer to your question is that portrait and landscape don't really impact image size considerations. It's about size and density. In that page you'll find that Android will attempt to create a decent image if it doesn't have one that fits the device. If you just want one set of images, choose images that look good for the middle size/density (i.e., normal screen size, 160 dpi, icon size of 48x48).
One tool you may be interested in is the Android Asset Studio. While it works best in the Chrome browser, you may get it to work in Firefox. It allows for the easy creation of alternate image sizes. Check it out here: