You can set the image's "location" property to control the position of the image in the report.
You can refer to this link: https://social.msdn.microsoft.com/Forums/en-US/ab181b43-c7e6-4d4a-9253-414a00c977c2/centering-a-database-image-on-a-rdlc-report?forum=aspsqlserverreporting.
Best regards,
Aniya