Generally, any reports generated in Web Report Studio, even with a stored process, are generated in SASReport XML format. I thought you needed to register your logo and other graphics in the Metadata in order to use them in a WRS report. So I would not expect ODS ESCAPECHAR with POSTIMAGE/PREIMAGE to work in a WRS report.
Also, It was my understanding that the "Print to PDF" feature of WRS Reports was using a different capability to create the PDF file other than ODS PDF. My understanding was that the WRS SASReport XML was being converted to PDF using Java techniques and under that scenario, it would be entirely possible for a PDF version of a WRS report to look different from an ODS PDF report generated in Base SAS.
I think your best bet for help with this question might be to work with Tech Support.