Your website can’t support all mobile devices, but how to decide

You could use a device description database (such as WURFL) which will recognise the client device from the request headers. You can then query that database to decide if the device can handle your site (e.g. support javascript, or is the screen big enough) before deciding whether to redirect them to a different site.

You don't mention your environment, but WURFL supplies APIs for Java and PHP, and maybe others as well. If there's no supplied API, you can still use WURFL, but you'll have to parse and process the XML data yourself.

For Android, I think the best place for you to start would be here, it has a lot of information including standard screen sizes and how to display images while keeping them in the best possible quality.

http://developer.android.com/guide/practices/screens_support.html

I'd also suggest doing as much image manipulation as possible on your server. Images are a pain to work with on Android due to memory constraints and fragmentation. Two phones may store pictures taken the same way with different orientations, and there is no simple way to handle rotations, though it can be done (thankfully, I've yet to encounter a phone that incorrectly records Exif data, but I wouldn't be surprised if they existed...). The more you rely on the phone to do, the more chances you have for error due to manufacturers putting wrappers around and otherwise customizing how it handles media.

As for how to display, ideally if your back end is already doing a bunch of different resizes, you can include your screen density when you request the images and send the best size based on the dev guide. If you want to keep differences to a minimum, at least support med or high density for phones, and extra high density for tablets.

Just my two cents, I'm sure you'll have a lot of opinions. Good luck.