I just tested the border and it's fine
Yes, my UI sucks I know. (It's the minimap that got the border)
Turns out it ate the rest of the post...
Here's the location of the texture correctly (In my case)
Interface\\AddOns\\LookInTehCorner\\media\\glassborder
So you do :
Interface\\AddOns\\myTextures\\glassborder as suggested
Should work fine.