The RoleTailored client includes images that you can use on actions that display in the Action Pane and command bar menus. To add an image to an action, you must provide the name of the image. However, it becomes difficult to choose the right image (from the long list) as there is no preview available.
I recently came across this useful link in Mibuso. There is an updated tool provided by Arend-Jan Kauffmann called Dynamics NAV Image Library. You can simply choose the version of NAV you are using and can view all the images. You can then choose the related name for use.
You can download the tool here. A good tool for the developers.