Hi Friend,
It depends on which feature set you want for your router for example latest image available for 2811 with advance enterprise services feature set will be like "c2800nm-adventerprisek9-mz.12.4-6.XE2.bin"
You can go to feature navigator on this link
http://tools.cisco.com/ITDIT/CFN/jsp/index.jsp
You need a CCO login to access the same.
Then you can click on Search By Platform and then from the dropdown select the product and then in next page you can select the main release and then the feature set and it will give you the name of the image.
HTH
Ankur
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