How to set height and width for external image in rdlc dynamically

I am using visual studio 2008, I have created a rdlc report in which i have an image in the header part. i want to set the height and width of that image dynamically as they vary frequently, please help me to set the image height and width dynamically or its better if someone can tell me how to set them to auto. suggest me if it is possible to set the height and width with the help of report parameters.