I suggest using the Imagetool that comes with the SDK (P3D SDK is free to download, and the tools works also with FSX).
The original image must be in RGB 24 bit and CAN (or not) have an Alpha channel, this depends if you want your logo to be on "your" white background, or you want it to blend with the underlying color of the logo panel, which usually looks better.
- Save the original file in .TGA, which is the easier format to work with and can be either 24 bit (RGB with no Alpha), or 32 bit (RGB + Alpha)
- Open it in the Imagetool.exe from the SDK
- Select the Image->Create Mipmaps from the menu
- Select the Image->Format to change the file format. If the original image has an Alpha channel, you should select DXT5, otherwise choose DXT1.
- Save the texture as a .DDS