From your description, you want to control the width and height base on the values of paramters, right? The width and height of the report items are not support expression in Reporting Services currently. So we cannot control the width and height of the
image at the run time dynamically. And I am afraid there is no other approch to work around this issue. If you have any concerns about this feature, you can submit a feedback at
http://connect.microsoft.com/SQLServer/Feedback and hope it is resolved in the next release of service pack or product.