Dynamic images on PDF via Reporting Services?

I have images stored on my server and I wish to take a particular image, based upon the data retrieved from my SQL Server 2005 database, and have it display on a PDF report that is being created by Reporting Services. I've seen recommendations that suggest this is how the overall process should be done - i.e. do NOT store the images in the database.  However, I cannot get the "Pointer" field from my report dataset to populate any image on the report.  I've tried full URL, partial URL, full file directory string, partial file directory string, with quotes, without quotes, but nothing seems to work.
Can anyone give a comprehensive answer to this please? Including wjhether or not I require quotes (single or double [or none])?
TIA.