You need to find an image that supports the NM-16ESW. If you have a few hours to spare, you could use the Cisco feature navigator to find one. http://tools.cisco.com/ITDIT/CFN/jsp/index.jsp
Or, you could just take the advice I give in the GNS3 Network Simulation Guide
You may have noticed that in the Switches dock of the Devices toolbar, there is an icon for an EtherSwitch router. This is nothing more than a Cisco 3725 router, preconfigured with a NM-16ESW card, which gives it 16 switch ports, quite independent from the router. The implication of this is that you MUST have an image for the c3725 router to be able to use the EtherSwitch device – I have found that the c3725-adventerprisek9-mz.124-15.T10 image works well with the EtherSwitch router because it supports the management of VLANs without having to resort to the vlan database commands.
And to locate the c3725-adventerprisek9-mz.124-15.T10 image on Cisco's website, I find it easier to do a google search for c3725-adventerprisek9-mz.124-15.T10.bin
rather than battle with the above mentioned feature navigator. You'll probably find that the first link takes you directly to the Cisco download (but of course you will need a maintenance contract with Cisco before you can download it)
Disclaimer: I am the author of the GNS3 Network Simulation Guide
-a VMware image of Ubuntu with GNS3 and VPCS installed and a collection of exercises/labs