The exact names of the icon files.
I wanted to change the standard nm-applet icons, so I created my icons in vector graphics. I replaced all standard icons with the extension .png with my icons with the extension .svg, but after launching nm-applet I got an error:
failed to load icon "nm-signal-75": Failed to load /usr/share/icons/hicolor/22x22/apps/nm-signal-75.png. I renamed all .svg to .png and it worked, the icons started to display, there were no errors in the terminal. But I think it would be more correct to change the paths and make them less accurate: instead of nm-device-wired.png, write nm-device-wired.*, etc.