As you can see in the screenshots the tooltip texture is split into 8 parts, each 16x16 pixel in size, thuse 128x16px in full size. This will make it possible to calculate graphics for every corner and every side.
Move the frame via SHIFT+LEFTCLICKDRAG.
Resize the frame via ALT+LEFTCLICKDRAG.
Frames that show a spell texture will stay at a square (looks ugly otherwise )
Why should I use SetBackdrop instead of SetTexture
The biggest plus of that is that your border will always be the same. It will not change it's width. So if you increase/decrease the size of a frame with that border, the border will stay constant.
If you use the trick of overlaying frame you can even create the illusion of a button. Lower layer will have a backdrop with an outer glow texture and a flat background. A layer above will have a different backdrop with just a border.