If you look at the RME Supported Devices Tables, it tells you that Image library import from device is not supported for the cat3550 switches.
You should be able to import the image from CCO.
Check out the following link for the Supported Device Table for Resource Manager Essentials 3.5 :
http://www.cisco.com/univercd/cc/td/doc/product/rtrmgmt/cw2000/cw2000e/dev_sup/index.htm
It states that "Software Management: Image library import from device, network baselining, and synchronization features are not supported." for Catalyst 3550 switches.