The copy/paste method certainly works, but having a lot more control over how the icon looks, it's status indicators, and how they are organized within the network atlas icon menu really helps make Network Atlas a sexier presentation tool.
First, create the icons. I built mine in Photoshop, and saved them as .gif. At the very least, you will need one iconName.gif, but providing iconName-Status.gif for each status allows you to create custom status indicators. If you leave this part out, you'll get the generic status flags instead.
Next, create a folder in the two directories listed below under File Locations, and paste your gif's into both directories.
Status options are (credit to Roger on this list from a previous thread):
Icon with no status: iconName.gif
Icon Up: iconName-Up.gif
In Orion 10.5, the files need to go in:
C:\Program Files (x86)\SolarWinds\Orion\Network Atlas\Maps\NetObjects\[FOLDER]
C:\Program Files (x86)\SolarWinds\Orion\Network Atlas\MapsWeb\NetObjects\[FOLDER]
Once the files are in place, you will be able to browse your [FOLDER] in Network Atlas when choosing a custom graphic, and they will be visible in the web console on your maps.
I'm using .gif files for this process, but I believe .wmf files will also work.