First, you'll want to authenticate using our tokening system. This requires a SOAP request to authenticate (HTTP Digest Authentication) and get a token back in the response which you can the stick in your URL for the Virtual Earth Map Control. There's a MSDN Technical Article on Implementing Customer Identification which you should review. There is even a version for Virtual Earth Supporting Files for Java to Implement Customer Identification.
Now the articles show you how to drop the token into the URL string, but you're hitting the image server URL directly, so you'll need to add the token id as a parameter in the image URL string like this:
Where tokenid is the token returned from Virtual Earth's authentication system.
So, let's say you get back the tokenID, 1234567. Your final image URL string for every image you're requesting would look something like this:
By adding this, you begin to conform to the terms of your contract, allow us to accurately track your usage (which could benefit your contract negotiations!) and allow you to accurately report on just how much traffic your site is REALLY getting and thus justify more budget to develop even cooler apps, Now, go make it happen.