I only know that the image is used on the outer circle part silver circle look at the pic from JappMe and I don't know if the same image is used on other areas of the UI.
I don't have Moria so I can't test further to see what else the image is used for.
I don't know if its even used for the gauge that you state.
So far I have that image invisible and no one has yet told me anything wierd about the gauge on my UI.
Or maybe I'm confusing what your talking about the guage.
Post a screenshot so I can see what your talking about?

I mean the "quickslot_buttonSlot.tga" image. It currently sits behind the Item Advancement button on the toolbar, but I have no idea what it does. I guess I'll make it invisible for now until we know what it's for.

For the Attunement gauge, it's the game mechanic used by the Rune-Keeper. I found out there's an option in the UI settings which lets you switch between the large and small versions.

Yeah I see,yeah like I said the only thing I see used for is to make that outer circle silver ring for the legendary button.
But yeah seems like a pointless image to be seperate from the rest of the main toolbar skin.
But who knows yet,it might be used on something under Moria UI panels.
But as far for non Moria client,I only see it use for that quick slot button on toolbar and no where else.