If you are going to tweak the runeblade graphic, the three files you'll have to replace are:
'runeblade'           (The standard background)
'runeblade-statusbar' (The bright runeblade that fills as power is gained)
'runeblade-proc'      (The background to display when Sudden Doom / Freezing Fog procs)

The name of the replaced files must be the same after replacing.

If the size of your background does not match up with rune button cooldowns, you can use the following two commands
to adjust the position and scrolling width of the rune buttons. The current settings make the buttons line up
just below the swordguard when ready and start at the tip of the blade when cooling down.

/runehero offset X (default 65)
/runehero width X  (default 260)

Know issue:
I don't have options yet to configure the status bar fill rate, so it will display oddly with modified textures 
Or you could just replace with a blank texture to disable.