I fell in love with gnome-netspeed-applet (http://projects.gnome.org/netspeed),
but I didn't like the icons at all. So I decided to change them.

Finding basic icon was easy. It was in ~/.icons/youricontheme/scalable/apps/gnome-fs-network.png
But the arrows were tricky to find, because most of my icon themes didn't have those icons...

So, I emailed to gnome-developer Jörgen Scheibengruber and got answer in couple of weeks:

> Hi,
> 
> if I remember correctly netspeed applet uses those icons from the current icon theme ("stock_navigate-next" and "stock_navigate
> -prev"). So you can either substitute these icons in your current icon theme or if you don't want to mess with that one 
> create a new icon theme that inherits your current one and just substitutes those two. Or you recompile netspeed and tell 
> it to use different icon-names, if you want to change netspeed only and no other apps.
> 
> Br,
> Jörgen

And so I did where to put my new icons: 
~/.icons/youricontheme/scalable/stock/stock_navigate-next.png
~/.icons/youricontheme/scalable/stock/stock_navigate-prev.png

...then killall gnome-panel,
and voilá!