Regarding images, if you added it into your public folder, I think there’s no need to perform assets:update
However, if you add the image within the resources folders of a bundle, you might have to, depending on your OS and which options you used when called assets:install
If you’re using an OS which supports symlinks (linux, OS X, and I guess all OS but Windows), you can install the assets calling (I don’t exactly remember the call, the important thing here is the symlink option):
php app/console assets:install web --symlink
This way, instead of having a copy of each bundle’s resources, you’ll have a symlink, so there should be no need to update. If you have an OS which doesn’t support symlinks, I think you’ll have to keep updating or reinstalling assets (in fact, I always used assets:install, I didn’t knew there was an update option :P).
Regarding the set up, I usually put all css, js, images and any public resources inside a bundle if it is used only within the bundle, and place it onto the public folder if it’s used by many bundles, or I plan to use it in other bundles.