Remember for SSRS reports that refer to external images (i.e. images not stored in a database or stored in the report itself), you need to prefix any UNC path with “file:” or the HTTP path with “http:”.
The full UNC path should be constructed like “file:\\server\share\path\filename.jpg”.
The full HTTP path should be constructed like “http:\\server\share\path\filename.jpg”
For UNC file paths be sure you have permissions for the SSRS service account and the Unattended account. You may want to consider pulling the files down to your local report server if performance issues prevail when the report retrieves images.