I ran into the problem with Firefox 5 where when I visited a web page using Silverlight that Firefox prompted me that the plugin would not work and I would need to restart in 32 bit mode. After the restart I would work but its a pain as each time I restarted I would be prompted to again.
Fluxify is a great Silverlight app that will resize your image and then email it to your favourite recipient. This is the only client size resize app that I can think of that can resize an image without first uploading and that doesn't require you to install it or assign special permissions.