Cisco IOS images for Dynamips - GNS3
http://docs.gns3.com/1-kBrTplBltp9P3P-AigoMzlDO-ISyL1h3bYpOl5Q8mQ/index.html#
An Overview of Cisco IOS Versions and Naming
http://www.ciscopress.com/articles/article.asp?p=2106547
Should I use a mainline and technology train image?
You will find the technology train images to contain the newest features but it may also contain the most bugs. We recommend Mainline over Technology train if you are interested on stability without the need of new features.Please see Cisco IOS Versions and Naming for more information.
Recommended images
We really recommend using c3640, c3660, c3725, c3745 and c7200 IOS images listed below, they have proven to be the most stable in GNS3 provided you use the right amount of RAM and Idle-PC value.
IOS 15 (Mainline)
This router series is still getting new IOS 15.x versions.
The last one at this date is:
File name: c7200-adventerprisek9-mz.152-4.M7.bin
Minimum RAM: 512MB
https://software.cisco.com/download/release.html?mdfid=282188585&softwareid=280805680&release=15.2.4M11&flowid=820
Cisco Systems
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IOS Software-15.2.4M11
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ADVANCED IP SERVICES 
Release: 15.2.4M11
Release Date: 17/Oct/2016