The PlayerFrame uses the texture from the target frame, but flipped. The texture file is also cropped.
(taken from PlayerFrame.xml)
Code:
<Texture name="PlayerFrameTexture" file="Interface\TargetingFrame\UI-TargetingFrame">
<TexCoords left="1.0" right="0.09375" top="0" bottom="0.78125"/>
</Texture>
You need to either a) make the graphic in your texture match the dimensions of the graphic in UI-TargetingFrame.blp, or b) alter the texture coordinates on the player frame border texture.