We have an images folder on our web servers that we may publish via a CDN. Sometimes we append query-string like syntax to URL's to help us freshen content that has changed, even though it rarely does. Example:
/images/file.png?20090821
will URL's like this work with your average content-delivery-network?
Yes, We use Akamai, which keeps a cached copy of each distict url requested including the querystring. So the first request for /images/file.png?20090821 will go to the origin server. Requests there after for /images/file.png?20090821 will get the image from the Akamai servers. The next day, assuming the img src changes to /images/file.png?20090822, the first request will go to the origin server again.
You wouldn't have problem with CDN. However, you may have problem with browsers. Some browsers wouldn't cache any content with query string. Even though it may be faster to fetch the image from CDN but it will not be as fast as cached image. So you want do something like this,
/images/file.png/20090821
Our CDN provider also recommends a hash mechanism. When we publish our content, it adds a hash to the URL so you don't have to add the version yourself. Unfortunately, I don't know the details on how that magic is done.