It would be nice to have an import feature for icons. This would be a big time saver without the need to export to Visio or NTA and manually replace the generic icons.
Personally I would use the standard Cisco network icons -
Network Topology Icons
NTM v2.2 includes new icons and custom icon import. See: Network Topology Mapper (NTM) 2.2 Now Available!
I'm marking this feature request as implemented!