Rotating the image can be done in graphics package like
Gimp.
You can extract the WoW image (and code) files by following
these instructions. They create the BlizzardInterfaceArt and/or BlizzardInterfaceCode folders under you Wow folder.
You can convert the .blp files using
Blp2Png or
BlPng Converter.
Save the rotated images as .tga (the Gimp default settings is all you need) or convert them back to .blp.
Copy the rotated image to your addon folder before starting the game, otherwise you will get a green square).
Change the path to the image to Interface\\Addons\\your addon\\image file.
Edit: If you want to just flip the texture along the x-axis then Texture:SetTexCoord(1 ,0, 0, 1). To flip along the y-axis then Texture:SetTexCoord(0, 1, 1, 0). This is commonly used for a texture file that contains a single image.