Thanks for the suggestions guys.
I haven't stored the images in a folder location per se. I've just pasted the image directly into the workbook so it resides in the .xlsx file.
Within the same workbook (that has three tabs), I've inserted the same logo stored as jpg, gif, bmp and png and all of these render fine on the first and second tab but none of them render at all on the third tab (of the same workbook). This behaviour occurs in other workbooks as well (about 5% of these problematic workbooks have this behaviour). I've looked for clues in the name manager (excel named ranges) and the web log. The web log is quite cryptic unless you a java guru
[http-80-7] ERROR () com.ibm.cognos.tm1.web.services.TM1Service - Exception Error: null
Stack Trace: java.lang.reflect.InvocationTargetException
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:94)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:55)
at java.lang.reflect.Method.invoke(Method.java:619)
at org.directwebremoting.impl.CreatorModule$1.doFilter(CreatorModule.java:229)
at com.ibm.cognos.tm1.web.services.dwr.DwrServiceFilter.doFilter(DwrServiceFilter.java:49)
at org.directwebremoting.impl.CreatorModule$1.doFilter(CreatorModule.java:223)
at com.ibm.cognos.tm1.web.services.dwr.LoggingFilter$1.invoke(LoggingFilter.java:34)
at com.ibm.cognos.tm1.web.services.logging.ServiceLogging.logServiceCall(ServiceLogging.java:33)
at com.ibm.cognos.tm1.web.services.logging.ServiceLogging.monitorCall(ServiceLogging.java:102)
at com.ibm.cognos.tm1.web.services.dwr.LoggingFilter.doFilter(LoggingFilter.java:32)
at org.directwebremoting.impl.CreatorModule$1.doFilter(CreatorModule.java:223)
at org.directwebremoting.impl.CreatorModule.executeMethod(CreatorModule.java:241)
at org.directwebremoting.impl.DefaultRemoter.execute(DefaultRemoter.java:379)
at org.directwebremoting.impl.DefaultRemoter.execute(DefaultRemoter.java:332)
at org.directwebremoting.dwrp.BaseCallHandler.handle(BaseCallHandler.java:104)
at org.directwebremoting.servlet.UrlProcessor.handle(UrlProcessor.java:120)
at org.directwebremoting.servlet.DwrServlet.doPost(DwrServlet.java:141)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:643)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:723)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at com.ibm.cognos.tm1.web.ResponseHeaderFilter.doFilter(ResponseHeaderFilter.java:40)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at com.ibm.cognos.tm1.web.GzipFilter.doFilter(GzipFilter.java:56)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:233)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:191)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:127)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:103)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:293)
at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:861)
at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:606)
at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:489)
at java.lang.Thread.run(Thread.java:795)
Caused by: com.ibm.cognos.tm1.web.session.SessionNotFoundException: TM1ServerSession ff303446-6fbb-49ef-8507-8e84bf1ed04f not found
at com.ibm.cognos.tm1.web.session.TM1WebSession.getTM1ServerSession(TM1WebSession.java:61)
at com.ibm.cognos.tm1.web.services.TM1AppService.getServerSession(TM1AppService.java:650)
at com.ibm.cognos.tm1.web.services.TM1AppService.getServerTree(TM1AppService.java:416)
at com.ibm.cognos.tm1.web.services.TM1AppService.refreshNavigationTree(TM1AppService.java:381)
at com.ibm.cognos.tm1.web.services.TM1Service.refreshNavigationTree(TM1Service.java:232)
Thanks guys, any pointers are appreciated!