With the release of Telerik Reporting Q1 2007 we had introduced a bug in the Image rendering (which is part of the PDF export as well) that we fixed in the consequent Service Pack 1. Please make sure you are running the latest release of Telerik Reporting - Q3 2007 SP1 (v. 18.104.22.168).
If the above information does not help please open a separate support ticket and send us a simple report (and the data it displays) that illustrates the described behavior. Any further information that you consider helpful is appreciated too.
All the best,
the Telerik team