You can actually skip all of the corners if you are making a plain border. All you really need is the top, bottom, left and right.
You will need to expirament a bit with the graphic you use, odd things happen to it when it gets stretched to fill in the border.
If you want a border that is 2 pixles wide, use a 2x2 square for it and make sure all 4 pixles in the square are the exact same shade. Use that square as the graphic for the border.
The game does some very odd looking things to it, when it stretches the graphic to fill in the border, if all the pixles are not the same shade/color. =)
By looking at the pic of the sample in your post, it looks like you want a patern?
Due to the way the border graphics get stretched, there is no simple way to get a patern to work.
In my
Caleal2 UI, I have a simple scrollwork patern for the side borders of various windows. Due to the scrollwork border, and the way my UI was designed to fit together, I had to create custom border templates for each individual window. You can find all the templates defined in the XML for the chat window.
To get the patern to look proper, it needs to be very simple, and designed so that it can be broken into 3 parts, preferably that can overlap eachother without throwing the patern off.
Its been about 7 months since I did mine, so I would have to think about it a bit to explain it. I still have nightmares about how much of a MASSIVE pain in the arse it was though. I defined everything in the chat window XML file and ended up with a file that is 44kb vs the 2kb of the default, >60% of that was for the custom border templates...
I will leave you with a few tips that will come in very handy should you decide to try making a border with a patern.
I found that, on the side borders, if you use 3 tall graphics, the middle piece will get stretched or shrunk to fit between the top and bottom pieces.
If you shrink the window down short enough that it is shorter than the combined hight of the top and bottom pieces, the top piece will overlap the bottom piece.
Trying to do a patern that works properly on the top and bottom of resizable windows is futile and only leads to insanity...
Download my Caleal2 and Caleal1.4 UIs and see how I broke down the graphics for the borders in the templates, they are all in the EQUI_ChatWindow.xml file. Also log into the game and play with resizing chat windows in the Caleal2 UI. It should give you an idea of what the game does with the border graphics.
To my knowledge, my Caleal2 UI is the only one in existance that has resizable custom borders, with a patern, that work properly, it does start to look wierd if you make the window very tall though... Not a brag, just an indication of how insain/stupid I am. =)
Good luck...